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 nova 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.

Redundant setenvs for specific test envs are removed to prevent them
from overriding the PYTHONHASHSEED value.

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

Change-Id: Ib54364877a251db48c54dfdc43c503281ea1f04a

tox.ini

@@ -7,7 +7,10 @@ skipsdist = True
 sitepackages = True
 usedevelop = True
 install_command = pip install -U --force-reinstall {opts} {packages}
+# Note the hash seed is set to 0 until nova can be tested with a
+# random hash seed successfully.
 setenv = VIRTUAL_ENV={envdir}
+         PYTHONHASHSEED=0
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
@@ -28,13 +31,11 @@ commands =
   bash tools/config/generate_sample.sh -b . -p nova -o etc/nova
 
 [testenv:pylint]
-setenv = VIRTUAL_ENV={envdir}
 commands = bash tools/lintstack.sh
 
 [testenv:cover]
 # Also do not run test_coverage_ext tests while gathering coverage as those
 # tests conflict with coverage.
-setenv = VIRTUAL_ENV={envdir}
 commands =
   coverage erase
   python -m nova.openstack.common.lockutils python setup.py testr --coverage \