Force python2.6 during python 2.6 jobs.

tox runs setup.py sdist outside of the venv because it's crazy. Until
we get that fixed, we need to let it know what python to use.

Change-Id: I90dd245dfcc3361fac0205394d207af2b0072638
Reviewed-on: https://review.openstack.org/24218
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins