reproducible_common: add file

This file should grow later to a reusable python library to avoid
redundant code. For now it allows to run diffoscope both with and
withouth schroot and compare files and get diffoscope version.

Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Holger Levsen <holger@layer-acht.org>