[Subnet] PM can delete subnet's gateway port, but subnet still hold gatewayPortId

[cj-chung]

Issue:
PM can delete any ports through PM's DELETE api without notify related service manager, such as Subnet.
If the deleted port is a Subnet's gateway port (device_owner=router_interface), the subnet manager still hold the GatewayPortId.

Solution:
If PM got request to delete a port related to some other resource manager, PM needs to notify them.