Compute v2 service for Rackspace.

Remaining work:

- [x] Direct delegation.
 - [x] flavors
 - [x] images
 - [x] servers
- [x] Acceptance tests.
 - [x] flavors
 - [x] images
 - [x] servers
- [x] Unit tests.
 - [x] flavors
 - [x] images
 - [x] servers