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

This doesn't currently handle output at all, so this isn't clearly useful for that purpose.