Fixes Multiple External Networks issue with DVR

Fixes Multiple External Networks issue with DVR

Current L3 agents can support more than one
external network when configured properly.

On DVR routers, router-gateway-set was
returning a 500 error, when two external
networks were configured in the system.

The problem resides in the scheduler where the
bind_router is called twice when the
reschedule_router is called from update_router.

The _schedule_router binds the snat
and the qrouter with the respective agents.

But after scheduling it does not return agent.

And in the case of two external networks, the
get_candidates always returns a valid candidate
to be processed and hence the bind_router is
called twice.

This patch fixes the _schedule_router function
and hence avoids the multiple calls to
bind_router.

This prevents the update_router from failing
and causing the nested rollback for the
transactions.

Change-Id: I24d44c60a3ea5bbc9e3f44aa5191deff315723ca
Closes-Bug: #1374473