Add rax-admin extension to the project

This extension provides a place to put rackspace stuff that doesn't, or
can't be put anywhere else. For example, you can execute the method
self._quota_usage() with the following

```
curl -i http://cinder.rackspace.com/v1/{tenant_id}/rax-admin/action \
 -X POST -d '{"quota-usage": null}'
```