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

modules/openstack_project/files/jenkins_job_builder/config/pbr.yaml

@@ -21,6 +21,7 @@
           #!/bin/bash -xe
           export PYTHONUNBUFFERED=true
           export DEVSTACK_GATE_TEMPEST=0
+          export DEVSTACK_GATE_EXERCISES=0
           function post_test_hook {
               bash -xe $BASE/new/pbr/tools/integration.sh $PROJECTS $BASE
           }