Properly initialize rpc in cinder-volume-usage-audit

When RPC isn't initialized, any RPC based notifiers will fail, which
renders cinder-volume-usage-audit mostly useless.

Closes-Bug: #1316483
(cherry picked from commit e52445681400ff1f8a78a39dca07d4d9711dfe52)

Change-Id: Ib6b9b1fdc76f289e9a44de520eeccbf99e28f40b
Merge "Properly initialize rpc in cinder-volume-usage-audit" into stable/icehouse