Switch deprecated rabbit_notifier to rpc_notifier

The 'nova.notifier.rabbit_notifier' is deprecated since the F release of
Openstack in favor of 'nova.openstack.common.notifier.rabbit_notifier'.
The latter being deprecated since the G release of Openstack in favor of
'nova.openstack.common.notifier.rpc_notifier'.

Change-Id: I4bd5819fd8e1fc7c75b20273418d821a04c4858e