This PR fixes token revocation and adds tests to make sure it is
working. The 0.5.6 RC1's token revocation does not work becasue the
token's value is captured at the instantiation of the deferred
stoprenewal statement rather than its exectution.

This PR fixes an oversight in which the client would attempt to renew a
token even after the task exits.

Fixes https://github.com/hashicorp/nomad/issues/2475

This PR fixes an issue in which the reference count on a Docker image
would become inflated through task restarts.

Fixes #2289

Unfortunately took more RecoverableError hijinx than I would have liked.
There might be a better way.

This PR fixes an issue that is hit when running templates with restart
mode in which the client could panic when the handle is not running.

Fixes https://github.com/hashicorp/nomad/issues/2479

This PR adds handling for downloading git artifacts using ssh with the
format git@github.com:hashicorp/go-getter.git

Fixes https://github.com/hashicorp/nomad/issues/2430

If they're unmounted by a reboot they'll be properly remounted.

Fixes https://github.com/hashicorp/nomad/issues/2042

This PR fixes two issues:

1) A close of a nil stopCollection channel when restoring and prestart
fails. The failure will cause the killCh to be triggered which will
close collection before it has been initialized.

2) Fixes a deadlock in which the handleWaitCh is never triggered since
it is not initialized when there is an error in prestart and the killCh
is triggered.

Both fixes are by maintaining the loop invariant that the two channels
are valid after there is a handle.

This *might* be a fix for #2295 -- I've been unable to reproduce the
bug. However, this guard seems wise regardless. I should never be
overwriting an intialized created resources with a nil.

Combined with b522c472 this fixes #2256

Without these two commits in place upgrades to 0.5.3 panics.

TaskRunner handles retrying but Cleanup handles all of CreatedResources.

Cleanup can be used for cleaning up resources created by drivers to run
a task. Initially the Docker driver is the only user (to remove
downloaded images).

This PR makes us only try to collect stats once if the Driver doesn't
support collecting stats.

Fixes https://github.com/hashicorp/nomad/issues/1986

This PR causes driver events to be sent to the server immediately rather
than waiting for Prestart() to finish.

* Refactor AllocDir to have a TaskDir struct per task.
* Drivers expose filesystem isolation preference
* Fix lxc mounting of `secrets/`

The Driver.Prestart method currently does very little but lays the
foundation for where lifecycle plugins can interleave execution _after_
task environment setup but _before_ the task starts.

Currently Prestart does two things:

* Any driver specific task environment building
* Download Docker images

This change also attaches a TaskEvent emitter to Drivers, so they can
emit events during task initialization.