Begin work to modernize pbr's integration testing

PBR does integration testing by installing all of openstack's python
projects to ensure we can install all of them in a variety of different
ways with different pip versions. Unfortunately, these jobs are still
using devstack-gate. We can simplify them significantly because zuul is
now doing what devstack-gate does and provides repos configured the way
we want them to be.

This change updates the jobs to drop devstack-gate and simply consume
zuul's repo prep.

A follow up will remove the old legacy jobs as they need to be removed
from project-config first.

Needed-By: https://review.opendev.org/745189
Change-Id: I77a38c4611dc28db79d6f25d96ad0de36b224c98

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