Set python hash seed to 0 in tox.ini

New tox (>=1.7.0) sets a random python hash seed by default. This is
generally good for testing because it will help keep projects working
regardless of the hash seed, but designate unittests don't currently
pass with a random hash seed so set it to the python default seed.

This change will allow us to use new tox again and remove the
restriction on tox<=1.6.1 to run unittests.

Note this change will need to be backported to the stable branches to
keep unittests there working with new tox as well.

See also Ib54364877a251db48c54dfdc43c503281ea1f04a

Change-Id: I3b3d152148a2055da1e2eaef5f4fbb63ebbde6fa