[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

There is already another "implementation" [here](https://github.com/ClusterHQ/flocker/pull/1013/files#diff-ed10ba0fb57c8554a8bdb17329f1ee88R52).

I'm not a fan of having the implementation inline in lambda's like this, but I think separating the implementation from the description is worthwhile.

(What I *really* would like is to use pattern matching on an algebraic type here, but this is python).