Skip devstack exercises on pbr run.

Change-Id: I2e2e14cab9e45f0c9290212737a604b8cf2f3313
Reviewed-on: https://review.openstack.org/34044
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins