Right now we have seen some performance issues when
dvr routers are scheduled on multiple compute nodes
with thousands of VMs on the routed subnets.
The _get_candidates call get_l3_agent_candidates with
a complete list of agents irrespective of the routers
already hosted on the agents or not.
So this fix will reduce the amount of iterations that
get_l3_agent_candidates need to process for all the
agents and would increase the control plane performance.
Closes-Bug: #1513678
Change-Id: I8f781d4cbc996ce13441303c9296e4f6ec822b94