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

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

This adds a message that provides environment setup instructions for
running e2e tests after running terraform apply.

This allows copy/pasting exports, rather than manually constructing
them.

Use a dedicated /dev mount so we can inject more devices if necessary,
and avoid allowing a container to contaminate host /dev.

Follow up to https://github.com/hashicorp/nomad/pull/5143 - and fixes master.

upstream fixes for accept-loop error handling

Restore some `exec` behavior 

WIP: fix failing tests caused by async driver manager

Restores pre-0.9 behavior, where Nomad makes /dev available to exec
task.  Switching to libcontainer, we accidentally made only a small
subset available.

Here, we err on the side of preserving behavior of 0.8, instead of going
for the sensible route, where only a reasonable subset of devices is
mounted by default and user can opt to request more.

libcontainer based drivers (e.g. exec, java) should default to running
processes as `nobody` [1]; but libcontainer treats empty user as `root`
in our case (either because of default or due to `root` being current
user).

[1] https://github.com/hashicorp/nomad/blob/94c28a4c6cc45f1b377d50a6bff2017b46c43d18/website/source/docs/job-specification/task.html.md#task-parameters