Move py33 env before py2x

When running "tox" without specifying the environment, we want py33 to
run before py2x so testr creates its database using a format available
in both versions of python.

Change-Id: If068685116156f30a7c4fe1f3a88f1a3e1a2fe00
Import run_cross_tests.sh from oslo-incubator

Change-Id: Ia97748eea46503a22767960c5b37f29f2d620db8