Correct PBR's gate_hook function.

* modules/openstack_project/files/jenkins_job_builder/config/pbr.yaml:
Previously devstack-vm-gate.sh was run with bash -xe neither are
necessary and the -e makes the test fail when it shouldn't
(devstack-gate relies on internal non zero return codes). Remove these
bash options and run the PBR integration script with -xe instead.

Change-Id: If77e17af5be714ee2e3c529d3cfbed1ec7a5f5b2
Reviewed-on: https://review.openstack.org/36518
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins