Migrate away from rackspace_resource

Having Rackspace specific resources subclass a the rackspace_resource
does not replace the clients system wide. Use the cloud_backend to
acomplish this. That way any special configuration done with pyrax
will be used in any client call.
Change-Id: Ia6d548e606a42c608135cb66876419d1b339a06c
Merge "Migrate away from rackspace_resource"