[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).


Download I suppose, so that the tutorial image can be built in parallel from another buildstep while cloud acceptance tests are being run.
Also so that we can continue to provide a tutorial box download link in the buildbot output.
The buildbot tutorial image builder will eventually upload to vagrantcloud for tagged releases, right? Another argument for building it in a separate buildstep and downloading it here?