This PR improves how killing a task is handled. Before the kill function
directly orchestrated the killing and was only valid while the task was
running. The new behavior is to mark the desired state and wait for the
task runner to converge to that state.

drivers/exec: restrict devices exposed to tasks

Emit Killing task events and add e2e tests

test: re-eanble periodic fingerprint test

We were just emitting Killed/Terminated events before. In v0.8 we
emitted Killing/Killed, but lacked Terminated when explicitly stopping
a task. This change makes it so Terminated is always included, whether
explicitly stopping a task or it exiting on its own.

New output:

2019-01-04T14:58:51-08:00  Killed            Task successfully killed
2019-01-04T14:58:51-08:00  Terminated        Exit Code: 130, Signal: 2
2019-01-04T14:58:51-08:00  Killing           Sent interrupt
2019-01-04T14:58:51-08:00  Leader Task Dead  Leader Task in Group dead
2019-01-04T14:58:49-08:00  Started           Task started by client
2019-01-04T14:58:49-08:00  Task Setup        Building Task Directory
2019-01-04T14:58:49-08:00  Received          Task received by client

Old (v0.8.6) output:

2019-01-04T22:14:54Z  Killed            Task successfully killed
2019-01-04T22:14:54Z  Killing           Sent interrupt. Waiting 5s before force killing
2019-01-04T22:14:54Z  Leader Task Dead  Leader Task in Group dead
2019-01-04T22:14:53Z  Started           Task started by client
2019-01-04T22:14:53Z  Task Setup        Building Task Directory
2019-01-04T22:14:53Z  Received          Task received by client

Test fix races

deps: Update x/net/context

plugins: Shutdown internal plugins

This implements the InternalPluginDriver interface in each driver, and
calls the cancellation fn for their respective eventers.

This fixes a per task goroutine leak during test suite execution.

This allows us to correctly terminate internal state during runs of the
nomad test suite, e.g closing eventer contexts correctly.

This allows std context to be used in place of an x/net/context and
unblocks updating to the latest version of go-plugin.

Task runner recovers from external plugin exiting

fix for user-specified auto-join tags

taskenv: have maps take precedence over primitives

e2e: Add consultemplate test

This adds a basic test for consul template, that verifies the behaviour
of consul-template with task blocking and restarting of tasks

Standardize driver health description messages for all drivers

remove unnecessary logging in rkt driver fingerprint method

We ultimately decided to provide a limited set of devices in exec/java
drivers instead of all of host ones.  Pre-0.9, we made all host devices
available to exec tasks accidentally, yet most applications only use a
small subset, and this choice limits our ability to restrict/isolate GPU
and other devices.

Starting with 0.9, by default, we only provide the same subset of
devices Docker provides, and allow users to provide more devices as
needed on case-by-case basis.

This reverts commit 5805c64a.
This reverts commit ff9a4a17.

e2e: Output setup instructions after terraform

taskrunner: emit TaskReceived event

Preserve pre-0.9, where task runner emits `Received: Task received by
client` event on task runner creation.

updated to latest hashicorp/raft and hashicorp/memberlist to pull