Remove subnet_id from check_ports_exist_on_l3agent

Refactor check_ports_exist_on_l3agent so that subnet_id no longer
needs to be passed.  Instead it calls get_subnet_ids_on_router.  This
helps to pave the way for removing hints from schedule router.

Partial-bug: #1353266
Partial-bug: #1356639
Co-Authored-By: Swaminathan Vasudevan <swaminathan.vasudevan@hp.com>

Change-Id: I6e9dcb0b899294bb4cf3e3d616a0a690049c338e
Merge "Remove subnet_id from check_ports_exist_on_l3agent"