Adds rackspace cloudnetworks-us provider

Egads! I think I understand the underlying motivation, but adding magic api-version-rewriting-logic for all calls (rather than being some option that users have to switch on, or provide themselves by adding a custom module to their context) seems a bit scary.

Would it make more sense to allow users to specify an "alternative endpoint" to try for their context? Or is it infeasible to delegate this kind of logic to the user?