remove jammy-security duplication

merging security.ubuntu.com into the same entry with archive.ubuntu.com
slightly adapt the nginx.conf-gen.py to make the nginx.conf list all specified hosts

closes #3 (closed)

Note:
nginx uses the first server context as a default even when server_name directive doesn't match. So we could get rid of this behavior via an entry like

    server {
        listen      3142;
        return      444;
    }

but imo it's a good thing to have this default behavior to maybe catch some different ubuntu mirror URLs in the sources.list that we didnt define in the cfg.yaml, which would fail otherwise.