Cleanup and reorder dockerfile

Also changes how we set the docker apt repo
Add separate tempest start script and image

Creates a dockerfile specific to tempest runs,
adds tempest stuff to the base dockerfile as well
(tempest gets installed for interactive runs, it just doesn't run
as a CMD)
Cleaner prereq install - install tempest pip reqs

Use devstack's prereq install script instead of hard-coding in
all of the debian packages! This is a lot cleaner!

While tempest pip reqs should be included already in
global-requirements, it seems there must be an upstream bug.
This works around that.