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).
