Fix various inventory scripts to use env python

Fix https://github.com/ansible/ansible/issues/9948

plugins/inventory/cobbler.py

-#!/usr/bin/python
+#!/usr/bin/env python
 
 """
 Cobbler external inventory script

plugins/inventory/freeipa.py

-#!/usr/bin/python
+#!/usr/bin/env python
 
 import json
 from ipalib import api

plugins/inventory/gce.py

-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2013 Google Inc.
 #
 # This file is part of Ansible