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 tempest unittests don't currently
pass a 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.

Change-Id: Ib54364877a251db48c54dfdc43c503281ea1f04a
Merge "Set python hash seed to 0 in tox.ini"