Also skip Chown on allocdir migration on Windows and when non-root.
Windows doesn't support it, and it will always fail as a non-root user.

* alloc_runner

* Random tests

* parallel task_runner and no exec compatible check

* Parallel client

* Fail fast and use random ports

* Fix docker port mapping

* Make concurrent pull less timing dependant

* up parallel

* Fixes

* don't build chroots in parallel on travis

* Reduce parallelism on travis with lxc/rkt

* make java test app not run forever

* drop parallelism a little

* use docker ports that are out of the os's ephemeral port range

* Limit even more on travis

* rkt deadline

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

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).

* 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.

Since lxc support requires linking to a C lib at compile and runtime
I'm putting it behind a build flag to avoid forcing all nomad users to
install liblxc (lxc-dev for development).