roles/nginx: Return HTTP 444 for requests to invalid hostnames

444 is a special nginx return code that means the request was
closed without a response, see:

    http://nginx.org/en/docs/http/request_processing.html

roles/nginx/templates/blank-vhost.conf.j2

@@ -7,7 +7,7 @@ server {
     listen [::]:80 default;
     server_name _;
 
-    return 403;
+    return 444;
 }
 server {
     listen 443 ssl http2 default;
@@ -34,5 +34,5 @@ server {
     # of such infrastructure, consider turning off session tickets:
     ssl_session_tickets off;
 
-    return 403;
+    return 444;
 }