Adds rackspace cloudnetworks-us provider

It does not backspace over this, exactly. The service-side API will always have a v2.0 in the path.
However, the endpoint will sometimes contain a v2.0 and sometimes it will not.
The annotation here ensures the path is always added. The VersionAwareRegionToEndpoint ensures that the endpoint will always look the same.
It is a minimal hack to get things working until versioning can be improved in the KeystoneModule and base tests (my guess). 
For now, I will add more comments!