CI: Use python3 for openstack clients

This switches our test jobs to use python 3 for the installation of
OpenStack clients used to test the control plane.

Change-Id: I6bf0a315cef174c42a69db3a7d57f602b27ac92c
Related: blueprint python 3

tools/setup_gate.sh

@@ -10,7 +10,10 @@ GIT_PROJECT_DIR=$(mktemp -d)
 
 function setup_openstack_clients {
     # Prepare virtualenv for openstack deployment tests
-    virtualenv ~/openstackclient-venv
+    if [[ "debian" == $BASE_DISTRO ]]; then
+        sudo apt -y install python3-venv
+    fi
+    python3 -m venv ~/openstackclient-venv
     ~/openstackclient-venv/bin/pip install -U pip
     ~/openstackclient-venv/bin/pip install python-openstackclient
     ~/openstackclient-venv/bin/pip install python-heatclient