[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

any error cases we care about?  I guess if you supply bad values then it's just going to blow up somehow and it doesn't really matter how.  Maybe just verifying that if one of the steps fails the overall task does fail is worthwhile though?  It'd be better to fail early than pretend things got set up and then have an acceptance test fail in some random bogus way.