Adds rackspace cloudnetworks-us provider

An alternative way to implement this is to figure out how to handle version inconsistencies and version negotiation in a consistent way for all the services, including the cases when there are noticeable differences between devstack and provider implementations. This can probably be based, to an extent, on the version negotiation currently in use for glance, or similar.
For now, however, this will allow devstack-tested code to work with provider code. The endpoint URI will contain a /v2.0 in one case, and in the other, it will not.