Add old_server option to setup_server script.

[config] / buster / etc_files / old_server / etc / apt / sources.list

deb http://deb.debian.org/debian stretch main contrib non-free
deb http://deb.debian.org/debian-security/ stretch/updates main contrib non-free
deb http://deb.debian.org/debian stretch-updates main contrib non-free
deb http://ftp.debian.org/debian stretch-backports main contrib non-free