[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).

TODO:
- [x] instance metadata
- [x] underscores
- [x] error reporting
- [x] usage documentation (including sample config)
- [x] buildserver config

It probably demonstrates that it's fairly straightforward to write our own sphinx extension to do what we want and overcome the shortcomings of `wordish`, though.  (Nothing particularly actionable here at the moment though)