This reverts commit cb36f453.

Avoid libcontainer.Process trying to lookup the binary via $PATH as the
executor has already found where the binary is located.

I chose to make them more of integration tests since there's a lot more
plumbing involved. The internal implementation details of how we craft
task envs can now change and these tests will still properly assert the
task runtime environment is setup properly.

The test is sadly quite complicated and peeks into things (logmon's
reattach config) AR doesn't normally have access to.

However, I couldn't find another way of asserting logmon got cleaned up
without resorting to smaller unit tests. Smaller unit tests risk
re-implementing dependencies in an unrealistic way, so I opted for an
ugly integration test.

This commit is a significant change. TR.Run is now always executed, even
for terminal allocations. This was changed to allow TR.Run to cleanup
(run stop hooks) if a handle was recovered.

This is intended to handle the case of Nomad receiving a
DesiredStatus=Stop allocation update, persisting it, but crashing before
stopping AR/TR.

The commit also renames task runner hook data as it was very easy to
accidently set state on Requests instead of Responses using the old
field names.

When Vault token expires and task is restarted, emit `TaskRestartSignal`
similar to v0.8.7

From https://github.com/hashicorp/nomad/blob/v0.8.7/client/task_runner_test.go#L1427

From https://github.com/hashicorp/nomad/blob/v0.8.7/client/task_runner_test.go#L1352

From https://github.com/hashicorp/nomad/blob/v0.8.7/client/task_runner_test.go#L620

From https://github.com/hashicorp/nomad/blob/v0.8.7/client/task_runner_test.go#L1275

From https://github.com/hashicorp/nomad/blob/v0.8.7/client/task_runner_test.go#L1195

Fixes restarting on recoverable errors from StartTask.

Ports TestTaskRunner_Run_RecoverableStartError from 0.8 which discovered
the bug.

0.9.0beta2 contains a regression where artifact download errors would
not cause a task restart and instead immediately fail the task.

This restores the pre-0.9 behavior of retrying all artifact errors and
adds missing tests.

Adds a new helper and removes a duplicated test.

Co-Authored-By: schmichael <michael.schurter@gmail.com>

Co-Authored-By: schmichael <michael.schurter@gmail.com>

Broke ShutdownDelay but the test was timing dependent so it just
appeared flaky. Made the test slower so that it should never incorrectly
pass.

Also fix race conditions in the mock vault client.

Added ability to adjust the number of events the TaskRunner keeps as
there's no way to observe all events otherwise.

Task events differ slightly from 0.8 because 0.9 emits Terminated every
time a task exits instead of only when it exits on its own (not due to
restart or kill).

0.9 does not emit Killing/Killed for restarts like 0.8 which seems fine
as `Restart Signaled/Terminated/Restarting` is more descriptive.

Original v0.8 events emitted:
```
	expected := []string{
		"Received",
		"Task Setup",
		"Started",
		"Restart Signaled",
		"Killing",
		"Killed",
		"Restarting",
		"Started",
		"Restart Signaled",
		"Killing",
		"Killed",
		"Restarting",
		"Started",
		"Restart Signaled",
		"Killing",
		"Killed",
		"Not Restarting",
	}
```

Also fix signal error handling in mock_driver.

Porting test from 0.8 to 0.9.

Restore a pre-0.9 test that asserts Consul services are deregistered
before a task's shutdown delay.

The driver manager is modeled after the device manager and is started by the client.
It's responsible for handling driver lifecycle and reattachment state, as well as
processing the incomming fingerprint and task events from each driver. The mananger
exposes a method for registering event handlers for task events that is used by the
task runner to update the server when a task has been updated with an event.

Since driver fingerprinting has been implemented by the driver manager, it is no
longer needed in the fingerprint mananger and has been removed.

The old approach was incomplete. Hook env vars are now:

 * persisted and restored between agent restarts
 * deterministic (LWW if 2 hooks set the same key)

This PR introduces a device hook that retrieves the device mount
information for an allocation. It also updates the computed node class
computation to take into account devices.

TODO Fix the task runner unit test. The environment variable is being
lost even though it is being properly set in the prestart hook.

Client.Shutdown now blocks until all AllocRunners and TaskRunners have
exited their Run loops. Tasks are left running.

The error messages are really bad, but it's extremely difficult to
produce good error messages without the original HCL.

Also add missing SetDriverNetwork calls.