Run py33 tests before py2x

As with 1dc9e67290ecf20161bf4b03ed0884de160ff22f from oslo.config,
If the py2x tests run before the py33 ones testr creates a database
that py33 can't read.  Reordering them in tox.ini allows a plain
tox run to succeed.

Change-Id: I6e727f7c4c39a55b4ea9b0655a91c5e017940d29