releng: Add script to change product update site to stable

releng: Add script to change product update site to stable

Also add a parent script that executes both this script
and the script that sets the automatic update.

Change-Id: Ifd5d384900ec0fb58145c7bc735ed5e180c54a65
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/79411
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>