nginx: reduce caching of upstream hostnames

etc/nginx/villas.conf

+resolver 127.0.0.1 [::1]:5353 valid=30s;
+
 # If we receive X-Forwarded-Proto, pass it through; otherwise, pass along the
 # scheme used to connect to this server
 map $http_x_forwarded_proto $proxy_x_forwarded_proto {