Use /usr/bin/env python for virtualenv (#1025)

This helps with virtualenv although it can open
up another security hole if you end up using an
unexpected python interpreter.

Overall it seems to make sense to err on the side
of usability but it's good to be aware of security.

However, for the remaining utility scripts that require
python 2, we explicitly note this with #!/usr/bin/python2.

examples/linuxrouter.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 """
 linuxrouter.py: Example network with Linux IP router