nginx: return 204 with a useful message for the default site

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

roles/nginx/templates/sites/default.conf

@@ -7,6 +7,7 @@ server {
     listen [::]:80 default;
 
     location / {
-        return 404;
+        return 204 "This is {{ inventory_hostname }}. If you came here there is a misconfiguration somewhere. Please inform gruppo-sistemisti@liste.ubuntu-it.org about the issue.";
+        add_header Content-Type text/plain;
     }
 }