VPNaaS doesn't consider subnet interface or router gateway removal operation

I met one problem when fixing bug #1255442. On Routed service insertion implementation such as VPNaaS, there is router and subnet validation check before creating a vpnservice and also router in using check before deleting a router.

But it did not consider subnet interface or the router gateway removal operation.
Whether it is okay to put all checks into l3_db.py or have some other more appropriate way to handle the problem.