Add hacking check for use of LOG.audit

Commit 4dc37abc removes the few instances of LOG.audit that
were in Cinder.  Given that the plan is to remove LOG.audit messages
from OpenStack, I am adding this hacking check to ensure that such
messages do not sneak their way back into Cinder.

Unit tests are included with this change.

Change-Id: Icc416a68f958f60260f1c55af0d8605c95913bf1
Merge "Add hacking check for use of LOG.audit"