Revert "Temporarily ignore Ceilometer Python 2.6 job"

This reverts commit e64fabf490d1ac18eff5457867a2af54c8d133a5.

The inadvertantly retired mongodb artifacts have now been restored,
so the temporary non-voting status for ceilometer/py26 is no longer
required.

Change-Id: Idb646d793ad6f9d26ad8b70ce9e6082fe720f3f5
Merge "Revert "Temporarily ignore Ceilometer Python 2.6 job""