Revert "Force python2.6 during python 2.6 jobs."

This reverts commit 80610a86a9c998aa876ae79f81523e0423363084

Change-Id: I3de4134669ff972c60797393783dd1d02763feb7
Reviewed-on: https://review.openstack.org/24493
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins