L3 Agent: allow to automatically reschedule virtual routers

In case of L3 agent failure, allow to reschedule automatically a virtual
router.

It's another way to provide HA for virtual routers with medium downtime.
The best way stays in using keepalived + DVR features but this
parameters allows another way to provide HA.

Change-Id: I0527e4a368e2b90b83877d3c7861e86674f9ae8a