Add workaround to support tox 1.7.2

Change-Id: I58542f6e7dbbad98517c09b6176b9827f66dd1ff
Partial-Bug: 1348818

tox.ini

@@ -7,6 +7,9 @@ envlist = py26,py27,py33,pep8,docs,sample_config
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
+         # FIXME(dolph): overriding the hash seed with a constant is a
+         # workaround for bug 1348818
+         PYTHONHASHSEED=0
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'