[FLOC-1070] Acceptance test runner.

The tutorial Vagrantfile hard-codes the version of the tutorial base box, which means that it needs to be edited to run tests against other versions.

This provides a new Vagrantfile, and a test runner that invokes it. Hopefully, the new runner should also be able to deal with cloud VMs eventually too.
Well, there should never be a reason to run this as root. I guess if you run this as `/path/to/python admin/run-acceptance-tests`, then this might pick up a different `trial`. We could replace this with
``[sys.executable, '-c', 'from twisted.scripts.trial import run; run()`]``. I'm not sure if that is sensible, though.