Change rpc_backend for cinder controller and node

I changed the 'rpc_backend' configuration key for cinder
controller and storage node to use the short version supported
by oslo. Will address Juno version in separate patch.

Change-Id: Ia942e31ce3a8c0508e2b41a4ecee87586478e392
Partial-Bug: #1358752
Merge "Change rpc_backend for cinder controller and node" into stable/icehouse