networkTests: old bridge overwrite

New created network should overwrite old bridge with the same name.

Change-Id: Id219e58effff59a1d4636e7117867379079227fd
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30578
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>