Return from check_ports_exist_on_l3agent if no subnet found

Return from check_ports_exist_on_l3agent if no subnet found

The call to get_subnet_ids_on_router can return an empty list.
If the subnet_ids list is empty, the subsequent call to get
the ports on a subnet returns all ports.  If this occurs
when doing a remove_router_interface, the performance
of a remove_router_interface degrades significantly.  This change
returns immediately from check_ports_exist_on_l3agents if no
subnet is found.  A new unit test has been added to cover
the specific case of returning immediately without calling
get_ports when a remove_router_interface operation is performed.

Change-Id: I247d3bae152ab4f8ab7e00bd24d878eb08dca1ba
Closes-Bug: #1420032
Depends-On: I15bbf16fd4378c6431e9da8942d0968e7a012a91