DVR:Fix _notify_l3_agent_new_port for proper arp update

DVR:Fix _notify_l3_agent_new_port for proper arp update

Now with notifications coming from ml2 plugin on port create
and port update, it is worth fixing the existing _notify_
l3_agent_new_port for proper arp update and router scheduling.

Previously we have been sending arp update and calling router
scheduling for every update notification for service ports,
but now we can take necessary action only when required, since
the fix to update the arp and router scheduling was recently
done by sending the port info for every new port created.

When _notify_l3_agent_port_update is triggered, we check if the
original port host binding exists and if there is a change in
host binding with respect to the new port, then we go ahead and
reschedule the router on the new host and flush the arp entry.

Related-Bug: #1524020

Change-Id: Ifda623d5413b72bf80f38fba5c12a05a88bb7de5