Convert instance_usage_audit() periodic task to objects

This makes the instance_usage_audit() periodic task use the
InstanceList object to get the list of instances in the window
instead of using a raw conductor api.

Related to blueprint compute-manager-objects-juno

Change-Id: Ib267d742e26b2d4285003619c5e5dedae8ec1035
Merge "Convert instance_usage_audit() periodic task to objects"