Non-voting RHEL6 Python 2.6 unit test checks.

* .../jenkins_job_builder/config/python-jobs.yaml: Add a template
gate-{name}-python26-rhel6 job identical to gate-{name}-python26 but
with a rhel6 node label. Also add it to the python-jobs group.

* .../zuul/layout.yaml: Make gate-{name}-python26-rhel6 non-voting
and go to extra lengths to avoid stable branches earlier than folsom
for now. Add it to all projects calling gate-{name}-python26
directly.

Change-Id: I8da8b27b5b7062e628665533df4e289fa2448ba8
Reviewed-on: https://review.openstack.org/24364
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Dan Prince <dprince@redhat.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins