engine: Set gateway for any network

If a cluster supports multiple gateways, any network should have its
gateway set according to the capabilities reported by VDSM.

Change-Id: Ie6af773b35276371826104df3a8bb2115d39f383
Bug-Url: https://bugzilla.redhat.com/995011
Signed-off-by: Lior Vernia <lvernia@redhat.com>