Set volume usage audit period to not NoneType

The setting of NoneType causes cinder-volume-usage-audit to not start up
and throw a stacktrace. This sets the config default back to something
it can use.

Change-Id: I4a9751025486b77dd7cf45d0aaad02e2325055d4
Closes-Bug: #1320541
Merge "Set volume usage audit period to not NoneType"