Ensure router exists when auto_schedule_routers

Currently, the auto_schedule_routers() accepts parameter router_ids,
which may contain invalid router ids, since we've already filtered
them via plugin.get_routers(), we can directly use that safe object.

Closes-Bug: #1217998
Closes-Bug: #1210877

Change-Id: I6196f16cca65fee4e848173d0a0a10fde967195d
Merge "Ensure router exists when auto_schedule_routers"