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 horizon 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.

Also see Ib54364877a251db48c54dfdc43c503281ea1f04a.

Change-Id: I8834487e8c7bcdbeae8397c1a10a18873976ce61
Partial-Bug: #1348818
(cherry picked from commit b30072e47ead3625cbf29a3b2955cae19a8e7533)