backend/s3: Support DynamoDB, IAM, and STS endpoint configurations

remove approximate release date

* Upgrading to 2.0.0 of github.com/hashicorp/go-azure-helpers

* Support for authenticating using Azure CLI

* backend/azurerm: support for authenticating using the Azure CLI

don't modify argument slices

core: enhance service discovery

This PR improves the error handling so we can provide better feedback about any service discovery errors that occured.

Additionally it adds logic to test for specific versions when discovering a service using `service.vN`. This will enable more informational errors which can indicate any version incompatibilities.

[Website] adding six new community providers

command/format: Add tests for ResourceChange renderer

helper/schema: Fix setting a set in a list caused error

There were a couple spots where argument slices weren't being copied
before `append` was called, which could possibly modify the caller's
slice data.

Previously we were only doing this in the case where count wasn't set at
all.

This causes the output to include additional helpful context such as
the values of variables referenced in the config, etc. The output is in
the same format as normal Terraform CLI error output, though we don't
retain a source code cache in this codepath so it will not include a
source code snippet.

Was incorrectly printing out "PlanResourceChange" instead of "Close".

In early versions of Terraform where the interpolation language didn't
have any real list support, list brackets around a single string was the
signal to split the string on a special uuid separator to produce a list
just in time for processing, giving expressions like this:

    foo = ["${test_instance.foo.*.id}"]

Logically this is weird because it looks like it should produce a list
of lists of strings. When we added real list support in Terraform 0.7 we
retained support for this behavior by trimming off extra levels of list
during evaluation, and inadvertently continued relying on this notation
for correct type checking.

During the Terraform 0.10 line we fixed the type checker bugs (a few
remaining issues notwithstanding) so that it was finally possible to
use the more intuitive form:

    foo = "${test_instance.foo.*.id}"

...but we continued trimming off extra levels of list for backward
compatibility.

Terraform 0.12 finally removes that compatibility shim, causing redundant
list brackets to be interpreted as a list of lists.

This upgrade rule attempts to identify situations that are relying on the
old compatibility behavior and trim off the redundant extra brackets. It's
not possible to do this fully-generally using only static analysis, but
we can gather enough information through or partial type inference
mechanism here to deal with the most common situations automatically and
produce a TF-UPGRADE-TODO comment for more complex scenarios where the
user intent isn't decidable with only static analysis.

In particular, this handles by far the most common situation of wrapping
list brackets around a splat expression like the first example above.
After this and the other upgrade rules are applied, the first example
above will become:

    foo = test_instance.foo.*.id

By collecting information about the input variables during analysis, we
can return approximate type information for any references to those
variables in expressions.

Since Terraform 0.11 allowed maps of maps and lists of lists in certain
circumstances even though this was documented as forbidden, we
conservatively return collection types whose element types are unknown
here, which allows us to do shallow inference on them but will cause
us to get an incomplete result if any operations are performed on
elements of the list or map value.

Although we can't do fully-precise type inference with access only to a
single module's configuration, we can do some approximate inference using
some clues within the module along with our resource type schemas.

This depends on HCL's ability to pass through type information even if the
input values are unknown, mapping our partial input type information into
partial output type information by evaluating the same expressions.

This will allow us to do some upgrades that require dynamic analysis to
fully decide, by giving us three outcomes: needed, not needed, or unknown.
If it's unknown then that'll be our prompt to emit a warning for the user
to make a decision.

This actually seems to be a bug in the underlying cty Convert function
since converting to cty.DynamicPseudoType should always just return the
input verbatim, but it seems like it's actually converting unknown values
of any type to be cty.DynamicVal, losing the type information.

We should eventually fix this in cty too, but having this extra check in
the Terraform layer is harmless and allows us to make progress without
context-switching.

This includes a change to improve the precision of types returned from
splat expressions when a source value is unknown.

Previously the test harness was preloading schemas from the providers
before running any test steps.

Since terraform.NewContext already deals with loading provider schemas,
we can instead just use the schemas it loaded for our shimming needs,
avoiding the need to reimplement the schema lookup behavior and thus
the need to create a throwaway provider instance with which to do it.

Previously we were running the factory function only once when
constructing the provider resolver, which means that all contexts created
from that resolver share the same provider instance.

Instead now we will call the given factory function once for each
instantiation, ensuring that each caller ends up with a separate object
as would be the case in real-world use.

backend/remote: use entitlements to select backends

This change enables a few related use cases:
* AWS has partitions outside Commercial, GovCloud (US), and China, which are the only endpoints automatically handled by the AWS Go SDK. DynamoDB locking and credential verification can not currently be enabled in those regions.
* Allows usage of any DynamoDB-compatible API for state locking
* Allows usage of any IAM/STS-compatible API for credential verification

This ensures that we test using the same source as we're using everywhere
else, and more tactically also ensures that when running in Travis-CI we
won't try to download all of the dependencies of Terraform during this
test.

In the long run we will look for a more global solution to this, rather
than adding this to all of our embedded "go" command calls directly, but
this is intended as a low-risk solution to get the build working again in
the mean time.

go plugin update

Fix provider import tests

In 98c8ac08 I merged a change to the vendored code for this module but
didn't spot that it didn't also update the dependency metadata to match.
Here we just catch up the metadata to match the vendored version, with
no change to the vendored code itself.

[Website] Skytap provider links