I believe I ended up mixing up permissions at /etc/ssl directories tree as the last modification was made on 18th November and a day after I could not get my PostgreSQL to work.
Check the output of
$ sudo -u postgres $ cd /etc/ssl/private $ ls
If the response is "Permission denied" do
$ chown postgres:ssl-cert /etc/ssl/private/ $ chown postgres:postgres /etc/ssl/private/ssl-cert-snakeoil.key