Get software-properties-gtk starting / fix /etc/os-release

Initial report

Originally created by @Anonymous on #12372 (Redmine)

This prevents GNOME Software to start on Tails/Stretch

This also breaks the configuration of repositories in Synaptic when software-properties-gtk is installed. It is included by default in Stretch with GNOME according to #11110 (comment 58005) but not in Tails.