On this branch some tests are still running under Python 2 and thus
rely on virtualenv (instead of us standardising on "python3 -m venv"
on the master branch where we are all Python 3). Use the zuul-role to
ensure the system package that provides "/usr/bin/virtualenv" is
installed.
Change-Id: I2b092bd70a9dbc6ef3a673ba45ca6925f9220bce
(cherry picked from commit 23cfb9e6eb)