lift code from docker/volume/mounts for splitting windows volumes

Use init() to handle plugin invocation

Update libcontainer to 6cc5158

Or github.com/opencontainers/selinux/go-selinux, a dependency of
libcontainer.

To pick up the fix in https://github.com/opencontainers/runc/pull/2023

Fixes https://github.com/hashicorp/nomad/issues/5836

metrics: add namespace label to allocation metrics

roll back to previous ami

Using the API as provided from the `mounts` package imposes validation
on the `src:dest` which shouldn't be performed at this time. To workaround
that lift the internal code from that library required to only perform
the split.

As with #5839 on Travis CI, this skips AppVeyor for UI branches.

Branches that follow the UI naming convention will no longer produce
non-UI jobs on Travis CI.

I failed to amend my commit! 😭

Mark rescheduled allocs as stopped.

docs: initial attempt at developer docs

Without this, clicking on the sort arrow didn’t change the
sort direction.

Currently, nomad "plugin" processes (e.g. executor, logmon, docker_logger) are started as CLI
commands to be handled by command CLI framework.  Plugin launchers use
`discover.NomadBinary()` to identify the binary and start it.

This has few downsides: The trivial one is that when running tests, one
must re-compile the nomad binary as the tests need to invoke the nomad
executable to start plugin.  This is frequently overlooked, resulting in
puzzlement.

The more significant issue with `executor` in particular is in relation
to external driver:

* Plugin must identify the path of invoking nomad binary, which is not
trivial; `discvoer.NomadBinary()` now returns the path to the plugin
rather than to nomad, preventing external drivers from launching
executors.

* The external driver may get a different version of executor than it
expects (specially if we make a binary incompatible change in future).

This commit addresses both downside by having the plugin invocation
handling through an `init()` call, similar to how libcontainer init
handler is done in [1] and recommened by libcontainer [2].  `init()`
will be invoked and handled properly in tests and external drivers.

For external drivers, this change will cause external drivers to launch
the executor that's compiled against.

There a are a couple of downsides to this approach:
* These specific packages (i.e executor, logmon, and dockerlog) need to
be careful in use of `init()`, package initializers.  Must avoid having
command execution rely on any other init in the package.  I prefixed
files with `z_` (golang processes files in lexical order), but ensured
we don't depend on order.
* The command handling is spread in multiple packages making it a bit
less obvious how plugin starts are handled.

[1] drivers/shared/executor/libcontainer_nsenter_linux.go
[2] https://github.com/opencontainers/runc/tree/eb4aeed24ffbf8e2d740fafea39d91faa0ee84d0/libcontainer#using-libcontainer

Update golang to 1.11.11

backfill region from hcl for jobUpdate and jobPlan

- updated region in job metadata that gets persisted to nomad datastore
- fixed many unrelated unit tests that used an invalid region value
(they previously passed because hcl wasn't getting picked up and
the job would default to global region)

Fix incorrect link to the sample Grafana dashboard

consul: Include port-label in service registration

It's the latest 1.11 point release as of now

Updated minimum development go version

When an alloc is due to be rescheduleLater, it goes through the
reconciler twice: once to be ignored with a follow up evals, and once
again when processing the follow up eval where they appear as
rescheduleNow.

Here, we ignore them in the first run and mark them as stopped in second
iteration; rather than stop them twice.