DVR: notify specific agent when creating floating ip

DVR: notify specific agent when creating floating ip

Currently when floating ip is created, a lot of useless action
is happening: floating ip router is scheduled, all l3 agents where
router is scheduled are notified about router update, all agents
request full router info from server. All this becomes a big
performance problem at scale with lots of compute nodes.

In fact on (associated) Floating IP creation we really need
to notify specific l3 agent on compute node where associated
VM port is located and do not need to schedule router and
bother other agents where rourter is scheduled. This should
significally decrease unneeded load on neutron server at scale.

Partial-Bug: #1486828
Change-Id: I0cbe8c51c3714e6cbdc48ca37135b783f8014905