[FLOC-965] Acceptance testing against rackspace.

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).


I agree. Can't we remove all the leading underscores? Elsewhere we've said that we're not currently promising any backwards compatibility guarantees and that we're not writing a library. So why  bother with the leading underscores?
Or maybe just make ``flocker._provision`` a private package...for now.