The logging package is used by logmon and the legacy mock_driver. Because the
legacy drivers are going away, I'm moving it here to signify its actual
ownership.

This creates a drivers/shared/structs package and moves the buffer size
checks into it.

As part of deprecating legacy drivers, we're moving the env package to a
new drivers/shared tree, as it is used by the modern docker and rkt
driver packages, and is useful for 3rd party plugins.

change to testing utilities to fix rkt tests

add generic plugin manager interface and orchestration

testing: in MkAllocDir, do not update TaskConfig with All() from the task builder, just with Env() (because it pollutes environment variables with node attributes and fails the rkt tests)

Modify fingerprint interface to use typed attribute struct

Device hook and devices affect computed node class

Add LXC driver

Looking at NewTaskRunner I'm unsure whether TaskRunner.TaskResources
(from which req.TaskResources is set) is intended to be nil at times or
if the TODO in NewTaskRunner is intended to ensure it is always non-nil.

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.

Fix deadlock in previous alloc watcher by emitting last alloc update

drivers/rkt: fix invalid volumes

taskrunner: Return the restart delay correctly