Correct the fake implementation of UsageVerifier
Correct the fake implementation of UsageVerifier

* rename clear_event to clear_events

Fixes: Bug #1234739
Change-Id: Ia3e2e9ecc85c49321793a098f272982bffc2230c

mmasaki/trove,changsimon/trove,mmasaki/trove,denismakogon/trove-guestagent,redhat-openstack/trove,cp16net/trove,fabian4/trove,zhujzhuo/openstack-trove,cp16net/trove,hplustree/trove,zhujzhuo/openstack-trove,zhangg/trove,fabian4/trove,zhujzhuo/openstack-trove,cp16net/trove,hplustree/trove,zhangg/trove,mmasaki/trove,openstack/trove,openstack/trove,redhat-openstack/trove,denismakogon/trove-guestagent,redhat-openstack/trove,fabian4/trove,changsimon/trove,changsimon/trove