Remove --system-site-packages from clients virtualenv

Using --system-site-packages here is causing us to use old versions of
libraries even though the correct version is installed into the
virtualenv, this is due to the order of the various directories in
sys.path. This is not a problem on newer versions of
virtualenv/setuptools although I couldn't find out exactly when it was
fixed.

In this case the specific conflict was with python-six. A recent
unrelated commit caused us to stop pip upgrading the version installed
from a package. In turn breaking several of the virtualenv installed
packages.

Change-Id: I030b57c1c3ece3772028ad08cc506c39bb8f3e21
Merge "Remove --system-site-packages from clients virtualenv"