Generate the .gitlab-ci.yml from a python script

This sits on top of !24 (merged), only the last three commits matter.

While working on the meson conversion and realising i'm going to have to build everything once with autotools and once with meson on every distribution, I figured it's easier to just generate those bits. So here it is, a python script that spits out all the various distribution:version@container-prep jobs and friends.

cc @bentiss