upgrade/I.1.2.0: manage L3 agent upgrades with short downtime

Migrating routers on L3 agent cannot be done without downtime, but at
least we can manage to migrate routers on other alive l3 agent before
rebooting a network node.
It will avoid long downtime (time to reboot against some seconds).

The migration is done by a Python script stolen from stackforge/cookbook-openstack-network and called from a bash script.

It's managed on network & full roles.