Add back rpc_notifier when ceilometer is enabled

This is to reverse what looks like collateral damage from change id
Ic375272b751159a64777ca73c1b64515195aacfb.

When the Ceilometer service is enabled, we also need to tell nova what to use to send
notifications.

Change-Id: I0015194cfa819e89ef85eae5020fedd6e7d71894
Closes-Bug: #1231158
Merge "Add back rpc_notifier when ceilometer is enabled"