After a bunch of recent changes/rebasing our vendored dependencies got a
little out of sync w.r.t transitive dependencies through codebases that
are not themselves Go Modules yet.

Also updates github.com/aws/aws-sdk-go and adds github.com/beevik/etree.

Fixes terraform-providers/terraform-provider-aws#3129.

Here we upgrade the AWS Go SDK to 1.12.27 and AWS provider to include terraform-providers/terraform-provider-aws#1608. 

This includes the capability to use named credentials profiles from the `~/.aws/credentials` file to authenticate to the backend.

The "terraform" provider was previously split out into its own repository,
but that turned out to be a mistake due to how tightly it depends on
aspects of Terraform Core.

Here we prepare to bring it back into the core repository by reorganizing
the directory layout to conform with what's expected there.

There is 1 breaking change. The OpsWorks Update API no longer supports
`tenancy` as an update. So that has to be marked as ForceNew: true

```
% make testacc TEST=./builtin/providers/aws
% TESTARGS='-run=TestAccAWSOpsworksInstance'
% 2 ↵ ✹ ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/02 17:11:29 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSOpsworksInstance -timeout 120m
=== RUN   TestAccAWSOpsworksInstance
--- PASS: TestAccAWSOpsworksInstance (99.89s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws99.911s
```

Update to v1.3.1 tag

This will allow for new changes to the ECS

vendor: Updated github.com/aws/aws-sdk-go to v1.1.23 The new STS library includes GetCallerIdentity which can be used to build ARNs for RDS resources when using IAM roles

We had actually caught mismatched deps versions between the `kms`
package and others, so `godep restore` was not working on master.

Fixing it requires a `godep update` so I figured we might as well update
to the latest release.

 * Remove `make updatedeps` from Travis build. We'll follow up with more
   specific plans around dependency updating in subsequent PRs.
 * Update all `make` targets to set `GO15VENDOREXPERIMENT=1` and to
   filter out `/vendor/` from `./...` where appropriate.
 * Temporarily remove `vet` from the `make test` target until we can
   figure out how to get it to not vet `vendor/`. (Initial
   experimentation failed to yield the proper incantation.)

Everything is pinned to current master, with the exception of:

 * Azure/azure-sdk-for-go which is pinned before the breaking change today
 * aws/aws-sdk-go which is pinned to the most recent tag

The documentation still needs to be updated, which we can do in a follow
up PR. The goal here is to unblock release.