Rewrite InstanceActionEvent object testcase using mock

This is a refactor change of rewriting InstanceActionEvent testcase
using mock for several follow-up changes to instance action objects.

Please Note 'test_finish' case against InstanceActionEvent object's
'finish' method has to be removed, as the action event object model
doesn't hold 'instance_uuid' information for now.

This is a part of: blueprint make-instance-action-use-objects

Change-Id: I0f4284995cd7afb4c5d1940a54996183be4eef8f
Merge "Rewrite InstanceActionEvent object testcase using mock"