set ssl_session_tickets to off by default

Necessary when other server{} blocks are defined in `/etc/nginx/conf.d/*.conf` without ssl_session_tickets set to off.
See https://community.letsencrypt.org/t/errors-from-browsers-with-ssl-session-tickets-off-nginx/18124/5?u=melwinkfr
Related to #580

nginx.tmpl

@@ -63,6 +63,9 @@ server {
 	return 503;
 }
 
+# Disable TLS Session resumption by default
+ssl_session_tickets off;
+
 {{ if (and (exists "/etc/nginx/certs/default.crt") (exists "/etc/nginx/certs/default.key")) }}
 server {
 	server_name _; # This is just an invalid value which will never trigger on a real hostname.