respect the virtualenv by using /usr/bin/env

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
diff --git a/external/keystone_user.py b/external/keystone_user.py
index 9290847..1245eec 100644
--- a/external/keystone_user.py
+++ b/external/keystone_user.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
 # Based on Jimmy Tang's implementation