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
diff --git a/tox.ini b/tox.ini
index 4f2465a..9c32121 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,23 +6,28 @@
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
          OS_TEST_PATH=./tempest/test_discover
+         PYTHONHASHSEED=0
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 
 [testenv:py26]
 setenv = OS_TEST_PATH=./tempest/tests
+         PYTHONHASHSEED=0
 commands = python setup.py test --slowest --testr-arg='tempest\.tests {posargs}'
 
 [testenv:py33]
 setenv = OS_TEST_PATH=./tempest/tests
+         PYTHONHASHSEED=0
 commands = python setup.py test --slowest --testr-arg='tempest\.tests {posargs}'
 
 [testenv:py27]
 setenv = OS_TEST_PATH=./tempest/tests
+         PYTHONHASHSEED=0
 commands = python setup.py test --slowest --testr-arg='tempest\.tests {posargs}'
 
 [testenv:cover]
 setenv = OS_TEST_PATH=./tempest/tests
+         PYTHONHASHSEED=0
 commands = python setup.py testr --coverage --testr-arg='tempest\.tests {posargs}'
 
 [testenv:all]