refactor(ssl): Moved /ssl to /certs

As this directory not only contains the SSL Certs,
the dkim certs as well, renamed to make more sense.

MR !2

include/etc/dovecot/conf.d/10-ssl.conf

@@ -11,7 +11,7 @@ ssl_prefer_server_ciphers = yes
 
 ssl_client_ca_dir = /etc/ssl/certs
 
-ssl_dh = </ssl/dovecot/dh.pem
-ssl_cert = </ssl/dovecot/cert.pem
-ssl_key = </ssl/dovecot/key.pem
+ssl_dh = </certs/dovecot/dh.pem
+ssl_cert = </certs/dovecot/cert.pem
+ssl_key = </certs/dovecot/key.pem