logmon fixes

Other logmon tests should be portable.

Port some integration tests of driver fingerprinting.

Some tests (e.g. `TestFingerprintManager_Run_DriversInBlacklist`) have
been subsituted by more isolated tests in
`client/pluginmanager/drivermanager/manager_test.go`

docker: Support Stats on Windows

This code chooses to be conservative as opposed to optimal: when failing
to reattach to logmon simply return a recoverable error instead of
immediately trying to restart logmon.

The recoverable error will cause the task's restart policy to be
applied and a new logmon will be launched upon restart.

Trying to do the optimal approach of simply starting a new logmon
requires error string comparison and should be tested against a task
actively logging to assert the behavior (are writes blocked? dropped?).

More alloc runner tests ported from 0.8.7

docker: Return undetected before first detection

This commit causes the docker driver to return undetected before it
first establishes a connection to the docker daemon.

This fixes a bug where hosts without docker installed would return as
unhealthy, rather than undetected.

Port some 0.8.7 alloc runner tests

Created .github; Added CoC

Set NextEval when making `failed-follow-up` evals

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

Port TestAllocRunner_RetryArtifact from https://github.com/hashicorp/nomad/blob/v0.8.7/client/alloc_runner_test.go#L610-L672

I changed the test name because it doesn't actually test that artifact
hooks is retried

Support json logging for CLI output for agent 

* caller should created eval id
* prev/next eval used in failed-follow-up

Do not wrap errors from StartTask as that loses Recoverable

There were multiple bugs here:

1. Reattach unmarshalling always returned an error because you can't
   unmarshal into a nil pointer.
2. The hook data wasn't being saved because it was put on the request
   struct, not the response struct.
3. The plugin configuration should only have reattach *or* a command
   set. Not both.
4. Setting Done=true meant the hook was never re-run on agent restart so
   reattaching was never attempted.

The initial fingerprint message is actually fairly useful, so I bumped
it to Debug and fixed the output formatting.

As far as I can tell this is the most straightforward and resilient way
to skip error logging on context cancellation with grpc streams. You
cannot compare the error against context.Canceled directly as it is of
type `*status.statusError`. The next best solution I found was:

```go
resp, err := stream.Recv()
if code, ok := err.(interface{ Code() code.Code }); ok {
	if code.Code == code.Canceled {
		return
	}
}
```

However I think checking ctx.Err() directly makes the code much easier
to read and is resilient against grpc API changes.

Fixes restarting on recoverable errors from StartTask.

Ports TestTaskRunner_Run_RecoverableStartError from 0.8 which discovered
the bug.

Fix regression by restarting on artifact download errors

* reformat lxc guide

* reformat lxc guide

docs: clarify use of toJSON for passwords

Run Docker tests in Windows AppVeyor CI