Drop python2 virtualenv for devstack

We no longer support python2, so we can just test using python3
virtualenv.

Change-Id: Id01f470e2d3424c3789a8f05f9eb05ac783f8363
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

devstack/plugin.sh

@@ -61,11 +61,7 @@ function install_glean {
 
 # Install nodepool code
 function install_nodepool {
-    if python3_enabled; then
-        VENV="virtualenv -p python${PYTHON3_VERSION}"
-    else
-        VENV="virtualenv -p python${PYTHON2_VERSION}"
-    fi
+    VENV="virtualenv -p python3"
     $VENV $NODEPOOL_INSTALL
     install_shade
     install_diskimage_builder