PY3: switch current build to use python3

PY3: switch current build to use python3

Make sure default make and configure for all now defaults
to building with python3.

To build a samba (or sub component e.g. talloc etc.) with python3
  ./configure && make

To build a samba (or sub component e.g. talloc etc.) with python2
  PYTHON=python ./configure && PYTHON=python make

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