New cells_rpcapi method get_host_availability_zone

Conflicts:
	nova/cells/manager.py
	nova/cells/messaging.py
	nova/cells/rpcapi.py

Change-Id: I12233ebdfacaab4fd0135fb7a7f5db0c36ce7c65
Child Cells send up available AZ's and last_seen
Support AZ api when using cells
AZ cell filter