use "/usr/bin/env python" for shebangs for python scripts

On NetBSD, a typical path of python interpreter is /usr/pkg/bin/python.

Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>
Acked-by: Ben Pfaff <blp@ovn.org>

utilities/ovs-pipegen.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#! /usr/bin/env python
 # Copyright (c) 2013, 2014, 2015 Nicira, Inc.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");