teach addNetwork to add a netwrok to an existing bridge

Although this adds an additional ossible flow for addNetwork,
this commit does not yet change VDSM's behavior as the situation
where a bridge should already exist should still not yet happen.

Change-Id: I0df074c6c9d5f846748c2cff2cc14ba24305123b
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35922
Reviewed-by: Dan Kenigsberg <danken@redhat.com>