Switching from RHEL to CentOS.

* .../jenkins_job_builder/config/python-bitrot-jobs.yaml: Add
centos6 bitrot variants of python26 bitrot jobs.

* .../jenkins_job_builder/config/python-jobs.yaml: Switch the node
label and name on python26 rhel6 jobs.

* .../zuul/layout.yaml: Replace python26 rhel6 job variants with the
new centos6 versions, but keep them non-voting still.

Change-Id: I31125b6c4d137591cf98716dbc8950537ad4d45b
Reviewed-on: https://review.openstack.org/27567
Reviewed-by: Dan Prince <dprince@redhat.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: matthew-wagoner <matthew.wagoner@hp.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