Change check_ports_exist_on_l3agent to pass the subnet_ids

Change check_ports_exist_on_l3agent to pass the subnet_ids

The get_subnet_ids_on_router is called for every
available l3agent in check_ports_exist_on_l3agent.
This introduces un-necessary call to the same
function multiple times which is expensive since it
calls get_ports internally.

In large scale the time taken to schedule a VM
on a given N-Node increases.

By passing the subnet_ids to check_ports_exist_on_l3agent
we would be only calling once get_subnet_ids_on_router in
the get_l3_agent_candidates.

This patch addresses the above problem by calling
get_subnet_ids_on_router just once.

Change-Id: I9d130f98e07bfe571eee32b827ff9af4010ff0fb
Related-Bug: #1513678