networkTests: assertBridgeDoesntExist added

New assert added, i need it in a following patch:
http://gerrit.ovirt.org/31018/

Change-Id: Ibf57fed6c320537a59e65b444184deb34d011048
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31017
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>