Prepare to test git-review

* .../files/jenkins_job_builder/config/projects.yaml: Switch the
python-jobs group in for the previous pep8 and tarball jobs, since
we want to start running pythonXX tests too.

* .../files/zuul/layout.yaml: Run voting gate-git-review-pythonXX
jobs. These are effectively no-op tests for the moment, but will be
shortly followed by actual integration tests already under review.

Change-Id: I8cd516eef55e25d200609f6de3f06d322df383c7
Merge "Prepare to test git-review"