Drop python26 support for Kilo nova

Now that we have stopped running python26 jobs in the gate, remove
python26 from the list of jobs in tox

project-config patch: Ib495467e00e4b6abf2b3c2b9445b51652ba25167

Change-Id: I18262e91c9f5fc2381c6f5b37eb45b5f27aa81db
Merge "Drop python26 support for Kilo nova"