Add adjustable offset to audit_period.
Add adjustable offset to audit_period.

Allows audit periods to be offset, so daily periods can begin at local
midnight, instead of UTC, monthly periods can begin on the 15th
instead of the 1st, etc.

Fixes bug 948601

Change-Id: I38f6af0a5a513f888b791a4b9ca39467030105f2

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects