We currently use an container image for `test-devices` job only; while
all other jobs use machine executor.

This allows us to switch golang and protoc verions easily without
manually managing Docker images (which requires building them manually
on a dev machines, etc).  All that while, we install dependencies on
every build in all other jobs..

`test-devices` now is one of the fastest jobs and isn't a constraint or
a bottleneck, so increasing its overhead by few seconds doesn't hurt the
overall developer iteration.

If we split tests effectively later, we can revisit.

Fixes `test-devices` job

Allow users to customize their makefiles by adding custom
targets/variables without checking them in.

Allow honoring `GO_TAGS` environment variable if set.  Currently, users
must set variable as a makefile argument e.g. `make GO_TAGS=ui dev`, and
this allows us to use env-var syntax (e.g. `GO_TAGS=ui make dev`) and
make it convenient to set GO_TAGS globally.

This commit provides an initial migration of general testing CI
infrastructure to CircleCI.

It uses CircleCI 2.1 paramereterised jobs to provide two base
configurations: a vm based `test-machine`, and docker based
`test-container`.

Jobs that require root, docker, or other similar features require the
machine based jobs, but others should be ran using the `test-container` package
as they are both cheaper and faster to run.

Our build scripts pass `$(GO_TAGS)` to `-tags` go compile flags, except
for `make dev`, where `$(NOMAD_UI_TAG)` is used.  This change ensures
`make dev` is inline with the rest of makefile targets.

I use the flag primarily to enable the nomad ui using the committed
compiled assets without regenerating them, as I find using stale ui
satisfactory most of the time.

`make check` runs very intensive linters that slow and seem to behave
differently on different machines.

Linting is still a part of our CI and we shouldn't be cutting a release
when CI isn't green anyway.

Noticed that the protobuf files are out of sync with ones generated by 1.2.0 protoc go plugin.

The cause for these files seem to be related to release processes, e.g. [0.9.0-beta1 preperation](https://github.com/hashicorp/nomad/commit/ecec3d38de6ca72eea68d9737d7199cf67bbfe3e#diff-da4da188ee496377d456025c2eab4e87), and [0.9.0-beta3 preperation](https://github.com/hashicorp/nomad/commit/b849d84f2fc8b47c5f5b0e387bff4fa09c3bca6e).

This restores the changes to that of the pinned protoc version and fails build if protobuf files are out of sync.  Sample failing Travis job is that of the first commit change: https://travis-ci.org/hashicorp/nomad/jobs/506285085

Gets us one step closer to a reproducible build. Also removes the unused
vendored version.

`travis.sh` contained some bash-ism despite it being invoked with `sh`.

`gotestsum` has user friendlier output that emits final summary, also it can emit junit xml file for
automated analysis instead of current format that should significantly
ease automated analysis of CI.

workaround a regression in 1.11.3

> We are aware of a functionality regression in "go get" when executed in GOPATH mode on an import path pattern containing "..." (e.g., "go get github.com/golang/pkg/..."), when downloading packages not already present in the GOPATH workspace. This is issue golang.org/issue/29241. It will be resolved in the next minor patch releases, Go 1.11.4 and Go 1.10.7, which we plan to release soon. We apologize for any disruption.

Lowering the runtime here to pre 7ca535aa expectations.

The longest package at the time `client/driver` shrunk significantly,
and now the longest packages take less than 5 minutes.

We do have some long running timed out projects due to a stuck shutdown,
but in completed jobs (though they failed), the longest packages took
less than 5 minutes.  The longest running packages in
https://travis-ci.org/hashicorp/nomad/jobs/464640776 were:

```
FAIL  github.com/hashicorp/nomad/nomad                                   268.089s
ok    github.com/hashicorp/nomad/drivers/docker                          203.903s  coverage:  68.8%   of  statements
ok    github.com/hashicorp/nomad/drivers/rkt                             132.104s  coverage:  65.0%   of  statements
ok    github.com/hashicorp/nomad/api                                     123.193s  coverage:  62.9%   of  statements
ok    github.com/hashicorp/nomad/command/agent                           74.657s   coverage:  72.3%   of  statements
ok    github.com/hashicorp/nomad/command                                 63.592s   coverage:  42.7%   of  statements
```

nomad/client take very long and exceed 15m sometimes:

In https://travis-ci.org/hashicorp/nomad/jobs/452990197 :

```
panic: test timed out after 15m0s

goroutine 4739 [running]:
testing.(*M).startAlarm.func1()
	/home/travis/.gimme/versions/go1.11.2.linux.amd64/src/testing/testing.go:1296 +0xfd
....
goroutine 4665 [select]:
github.com/hashicorp/nomad/vendor/google.golang.org/grpc.newClientStream.func5(0xc0003dd500, 0xc000420120, 0x2b3f86295588, 0xc000496810)
	/home/travis/gopath/src/github.com/hashicorp/nomad/vendor/google.golang.org/grpc/stream.go:287 +0xd7
created by github.com/hashicorp/nomad/vendor/google.golang.org/grpc.newClientStream
	/home/travis/gopath/src/github.com/hashicorp/nomad/vendor/google.golang.org/grpc/stream.go:286 +0x842
FAIL	github.com/hashicorp/nomad/client/driver	900.036s
```

Introduce a device manager that manages the lifecycle of device plugins
on the client. It fingerprints, collects stats, and forwards Reserve
requests to the correct plugin. The manager, also handles device plugins
failing and validates their output.

Allow it to fail for now with the goal of making it the default build in
the future.