This reverts commit c1dbf10562796c7006fd0a66c6cb39a1b990c8c1.
Reverting because it is valid to run devstack without enabling an rpc
backend. For example when you run devstack on a multinode deployment
compute hosts probably shouldn't run rabbit and instead will just talk
to the rabbit server set up on the controller node. As a result we
should not die when no rpc backends are set.
Change-Id: If9a31e031552f4161d42094fa960221b4d9e893c