Add add_aggregate_to_host() to FakeDriver

AggregatesSampleJsonTest.test_add_host fails if you enable CastToCall on
test_api_samples because FakeDriver doesn't have stub implementations
of add_aggregate_to_host() or remove_aggregate_from_host().

blueprint: oslo-messaging
Change-Id: I6089e174fc12222bd823ab59adf246190346a401
Merge "Add add_aggregate_to_host() to FakeDriver"