[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

To be clear, I like the extra information here not present elsewhere and I'd hate to lose it.  If it's too hard to fit into the `--help` format then a compromise might be to link to this document from the `--help` output (iow fix the duplication by making this canonical instead of making --help canonical)