python: Don't hardcode interpreter path

This is particularly useful on operating systems that don't ship
Python as part of the base system (eg. FreeBSD) while still working
just as well as it did before on Linux.

While at it, make it explicit that our scripts are only going to
work with Python 2, and remove the usage of unbuffered I/O, which
as far as I can tell has no effect on the output files.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

docs/apibuild.py

-#!/usr/bin/python -u
+#!/usr/bin/env python2
 #
 # This is the API builder, it parses the C sources and build the
 # API formal description in XML.

docs/index.py

-#!/usr/bin/python -u
+#!/usr/bin/env python2
 #
 # imports the API description and fills up a database with
 # name relevance to modules, functions or web pages

docs/reformat-news.py

-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # reformat-news.py: Reformat the NEWS file properly
 #

src/esx/esx_vi_generator.py

-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 #
 # esx_vi_generator.py: generates most of the SOAP type mapping code

src/hyperv/hyperv_wmi_generator.py

-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 #
 # hyperv_wmi_generator.py: generates most of the WMI type mapping code

tests/cputestdata/cpu-cpuid.py

-#!/usr/bin/python2
+#!/usr/bin/env python2
 
 import sys
 import json