Reschedule router if new external gateway is on other network

An L3 agent may be associated with just one external network.
If router's new external gateway is on other network then the router
needs to be rescheduled to the proper l3 agent

Change-Id: Ia0ed924403137ac4578ca562b57988292c41c1fe
Closes-Bug: #1234750
Merge "Reschedule router if new external gateway is on other network"