Switch jobs from Ubuntu Oneiric to CentOS 6.

Ubuntu Oneiric Ocelot will end of life on May 9, 2013. Jobs which
previously ran on oneiric unit test slaves because of availability
of Python 2.6 now run on centos6 slaves instead.

* .../jenkins_job_builder/config/heat.yaml: The dev-heat-coverage
and dev-python-heatclient-coverage previously now use centos6
slaves.

* .../jenkins_job_builder/config/python-bitrot-jobs.yaml: The
periodic-{name}-python26-{branch-name} template now uses centos6
slaves, and obsolete periodic-{name}-python26-{branch-name}-centos6
template is removed.

* .../jenkins_job_builder/config/python-jobs.yaml: The
gate-{name}-python26 template now uses centos6 slaves, and obsolete
gate-{name}-python26-centos6 template is removed.

* .../zuul/layout.yaml: Stop running the non-voting .*-centos6 jobs
as they're no longer necessary. Also stop running gate-nova-python26
on stable/folsom for now, as it won't be worth the effort to get
running successfully on CentOS at this point.

Change-Id: I29f0d6fca99c130aa9d9c5cfe9dfb0467dd10832
Reviewed-on: https://review.openstack.org/28252
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins