fail if devstack attempts to be run under virtualenv

This has come up on the mailing list recently, we should just fail
early and explicitly so that people don't get way down this path and
not realize it's never going to work.

Change-Id: I8a7f001adf3a5244b8655858ebd5fc7014a4af55

Author: sdague

stack.sh

@@ -93,6 +93,20 @@ if [[ $EUID -eq 0 ]]; then
     exit 1
 fi
 
+# OpenStack is designed to run at a system level, with system level
+# installation of python packages. It does not support running under a
+# virtual env, and will fail in really odd ways if you do this. Make
+# this explicit as it has come up on the mailing list.
+if [[ -n "$VIRTUAL_ENV" ]]; then
+    echo "You appear to be running under a python virtualenv."
+    echo "DevStack does not support this, as we my break the"
+    echo "virtualenv you are currently in by modifying "
+    echo "external system-level components the virtualenv relies on."
+    echo "We reccommend you use a separate virtual-machine if "
+    echo "you are worried about DevStack taking over your system."
+    exit 1
+fi
+
 # Provide a safety switch for devstack. If you do a lot of devstack,
 # on a lot of different environments, you sometimes run it on the
 # wrong box. This makes there be a way to prevent that.