test_engine_service remove redundant DummyThreadGroups

Remove DummyThreadGroup assignments, since they are assigning to an
EngineService attribute which no longer exists (it's a legacy from
an older heat-engine implementation)

Change-Id: I60de53e88eeb56ab36ec7f3a697fd856b1a84925
Merge "test_engine_service remove redundant DummyThreadGroups"