Cap virtualenv to a version < 20

There are errors with the recently released
virtualenv 20 for example: https://github.com/pypa/virtualenv/issues/1551

It has been capped on devstack side also
I3b447557f013934719aa357efeffb093c7f6bd95

Let's use the lower version and we can move to
latest once issues are fixed.

Change-Id: I341437856c709e1e0e281357b96901ae6ef7cd00

devstack-vm-gate-wrap.sh

@@ -500,7 +500,11 @@ else
 fi
 
 # Install ansible
-sudo -H $PIP install virtualenv
+
+# TODO(gmann): virtualenv 20.0.1 is broken, one known issue:
+# https://github.com/pypa/virtualenv/issues/1551
+# Once virtualenv is fixed we can use the latest one.
+sudo -H $PIP install "virtualenv<20.0.0"
 virtualenv -p python${PYTHON_VER} /tmp/ansible
 
 # Explicitly install pbr first as this will use pip rathat than