Temporarily ignore Ceilometer Python 2.6 job

* modules/openstack_project/files/zuul/layout.yaml: EPEL maintainers
inadvertently removed mongodb (confusing it with couchdb), which has
caused gate-ceilometer-python26 to fail. Temporarily switch it to
non-voting until mongodb is restored to EPEL or the relevant tests
are disabled in Ceilometer's source tree.

Change-Id: I89fd3e1740d72ef5f456914441daa0b4f43b061c