Fixes race condition and boosts the scheduling performance

Fixes race condition and boosts the scheduling performance

This patch fixes a race-condition that occurs when the
scheduler tries to check for dvr serviceable ports before
it schedules a router when a subnet is associated with
a router.

Sometimes the dhcp port creation is delayed and so the
router is not scheduled to the l3-agent.

Also it boosts the scheduling performance on dvr-snat
node for scheduling a router.

This patch will provide a work around to fix this race
condition and to boost the scheduling performance
by scheduling a router on a dvr-snat when
dhcp is enabled on the provided subnet, instead of checking
all the available ports on the subnet.

Closes-Bug: #1442494

Change-Id: I089fefdd8535bdc9ed90b3230438ab0bfb6aab4f
(cherry picked from commit c65d3ab6ad4589e6e4a6b488d2eb5d1e4cfee138)