[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).


What needs to be fixed here?  The hard-coded URL?  If so, what might cause one to want to use a different URL?