buildtools/wafsamba: re-write shebang for delivered python scripts

buildtools/wafsamba: re-write shebang for delivered python scripts

Can't see how the orig code would have worked though
a)
  task.env["PYTHON"] is a list
b) task.env["PYTHON_SPECIFIED"] can (and is in our case false) looks
   like it would only be true for python2 but in anycase no harm we
   always rewrite the shebang

So now it works as follows,

1. PYTHON (which is where the shebang is got) is set to python3 by default
2. To override the default you need to set PYTHON (e.g. to build with python2)
3. If you give a full path in PYTHON then shebang is of the format
   "#!{FULL_PYTHON_INTERPRETER_PATH)
4. If you specify PYTHON=python or PYTHON=python2 etc. shebang format is
   "#!!/usr/bin/env python", "#!!/usr/bin/env python2" etc.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>