Pass OS_* env vars fix for tox 2.0
Pass OS_* env vars fix for tox 2.0

Tox 2.0 stopped passing environment variables from the parent to
the tests, so the functional tests were failing since they rely
on keystone CLI getting the OS_* environment variables.

Change-Id: I4f05379df41a3f6ca24ce8eb6911144a381f058e

magic0704/python-keystoneclient,klmitch/python-keystoneclient,magic0704/python-keystoneclient,Mercador/python-keystoneclient,sdpp/python-keystoneclient,klmitch/python-keystoneclient,Mercador/python-keystoneclient,sdpp/python-keystoneclient