Rackspace client and Identity service

Refactor API key authentication into a Rackspace-specific extension of OpenStack identity v2. Along the way, bootstrap the Rackspace provider package with functions to create and authenticate clients.

## Progress

- [ ] Create the Rackspace identity service as a delegating wrapper around the OpenStack identity v2 service.
- [ ] Remove API key authentication from the OpenStack identity service. Move it into the Rackspace service.
- [ ] Write the `NewClient` and `AuthenticatedClient` functions to create a provider client that's authenticated through the Rackspace identity API.
 - [ ] *Possibly* do some refactoring in the utility methods in openstack/ to reduce the amount of copy-paste that's necessary.