Add availability_zone support for router

Add availability_zone support for router

This patch adds the availability_zone support for router.

APIImpact
DocImpact: Make router scheduler availability zone aware. If multiple
availability zones are used, set router_scheduler_driver =
neutron.scheduler.l3_agent_scheduler.AZLeastRoutersScheduler. This scheduler
selects agent depends on LeastRoutersScheduler logic within an availability
zone so that considers the weight of agent.

Change-Id: Id26d9494b9a5b459767e93a850f47a3b014b11bb
Co-Authored-By: IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
Partially-implements: blueprint add-availability-zone