relax the validation multiple networks on the same interface

the only validation that is now left on the vdsm side is that there are
no 2 networks using the same vlan tag that are connected to the same
interface concurrently. Also, no 2 untagged networks can be connected to
the same interface.

Change-Id: I0030433ef519ae6699ee8a921b95c0a67f7b2eae
Bug-Url: https://bugzilla.redhat.com/1049476
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34538
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>