[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

As discussed elsewhere, current version of libcloud has a bug that causes this to raise:

```
AttributeError: 'bool' object has no attribute 'lower'
```

Fortunately your fix was accepted by upstream.  Unfortunately it's not released yet...

So, something to fix here I guess, somehow.

Perhaps this is fortunate though, since it highlights another way cleanup can fail.  `create_node` has actually succeeded in the cases where it raises this exception - but the node isn't in `self.nodes` yet so it doesn't get cleaned up.

I don't know if it's possible to do the cleanup if `libcloud` won't give back a node instance :/  It would be nice to handle this case though (just in case libcloud has any more bugs).  At worst, maybe exceptions from here can be reported along with a warning that an instance may have just been leaked into the cloud.