Add zuul entries for pbr in the gate.

Change-Id: I155ce32a7866a2caf2ba2787579bdb9ba8c56840
Reviewed-on: https://review.openstack.org/28628
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Anita Kuno <anita.kuno@enovance.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins