[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

`print` probably isn't the right answer either though.  A better factoring would probably be for methods like `stop_nodes` to raise an exception and some other code that's closer to the CLI parts of this program do the output formatting.  Failing that, `self.output.write(...)` (or, in principle, `print >>self.output, ...` ... but please not that :wink:)