[FLOC-965] Acceptance testing against racksapce.

This is based on #1010. The relative diff is [here](https://github.com/ClusterHQ/flocker/compare/acceptance-runner-FLOC-1070...rackspace-acceptance-FLOC-965).

The linking tests fail due to [FLOC-1084](https://clusterhq.atlassian.net/browse/FLOC-1084).


It's probably eventually useful to be able to test things without getting buildbot involved - but I don't mind if there's a buildbot dependency for now if that's easier.  This sort of implies a lot of data transfer, unfortunately.  But perhaps that's true either way. :/  Even a small change to the code is going to require a totally new box... I suppose local build has the advantage that the base box gets to be cached.

I have trouble following what actually happens in `VagrantRunner.start_nodes`.  Where **does** the box come from?  Are you supposed to have downloaded it already?  Is it implicitly downloaded from vagrantcloud?