Use venv for VIRTUALENV_CMD

Using venv, which is part of python3, we avoid an extra dependency on
the virtualenv package.  For Debuntu, which splits this out into a
separate package, add this to debs/general.

This is part of the infra efforts to ship "plain" nodes without any
dependencies installed.  While devstack can re-install virtualenv, we
don't need any features it provides and it means one less dependency.

Change-Id: I3c323640f288e57581a4eb8adba2a08d0b0cbd8f
diff --git a/stackrc b/stackrc
index 4e33b68..17641c3 100644
--- a/stackrc
+++ b/stackrc
@@ -150,7 +150,8 @@
 
 # Create a virtualenv with this
 if [[ ${USE_PYTHON3} == True ]]; then
-    export VIRTUALENV_CMD="virtualenv -p python3"
+    # Use the built-in venv to avoid more dependencies
+    export VIRTUALENV_CMD="python3 -m venv"
 else
     export VIRTUALENV_CMD="virtualenv "
 fi