🚑 pottwal: Change group of prosody certs dir

prosody runs as unpriviledged user inside of its docker container with
this uid/gid:

    root@676f7272aaa6:/etc/prosody# id prosody
    uid=101(prosody) gid=102(prosody) groups=102(prosody),101(ssl-cert)

The certs dir has 0750 permissions and thus the process can not access
it and finds no certs, leading to stream errors for s2s and c2s
connections.  We can not use group name, because the same group has a
different gid on the host.  Numerical is fine (even if ansible needs
this as a string, `chown` seems to know how to handle this).

References: !119
Signed-off-by: Alexander Dahl <alex@netz39.de>

host-pottwal.yml

@@ -135,6 +135,8 @@
       ansible.builtin.file:
         path: "{{ prosody_config_dir }}/certs"
         state: directory
+        # group 'ssl-cert' inside of the container
+        group: "101"
         mode: 0750
 
     - name: Ensure prosody conf.avail dir exists