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.

Disallow stale queries when deriving Vault tokens

This PR disallows stale queries when deriving a Vault token. Allowing
stale queries could result in the allocation not existing on the server
that is servicing the request.

Make errors starting a container recoverable

This PR makes errors starting a container recoverable and tries to
optimistically handle 500 errors.

Returning a container if it exists instead of creating a new one

Correct a grammar typo

a "the a" was still on text, certainly after an hesitation from author.
Keep the "a" form, as it's more general.

Ensuring allocs are not added multiple times to blocking queue

Fixes an issue with purging containers with the same name Nomad is tr…

Fixes #2025

I forgot to validate the volumes field!

Fixes race on StreamFramer Destroy

Improve parsing of job struct

Log when lookup in docker.auth.config fails

Make container exist errors non-retriable

Use net.JoinHostPort instead of fmt.Sprintf

Using fmt.Sprintf breaks IPv6 addresses.

This change makes it so that the task runner does not retry container
exists errors and also a sleep is added on the local retry.

(bug)Fix error message. Pass on template args