Fix the device_id and device_owner of gw

This patch fix the values returned for the
gateway port of a router.
In order to be able to get the device_id
when doing a neutron port-show this patch
also add a new link in schema between
the service instance and the logical
router.

Change-Id: I98f8a268a3bae94e73e11ac40784009652faf848
Merge "Fix the device_id and device_owner of gw"