roles/reverse_proxy: fix http redirect with multiple server_names

When using multiples server names in `server_name`, the NGINX variable
`$server_name` will always point only to the first entry. `$host`
variable, instead, point to the selected host from the user (HTTP
request or Host: header).

roles/reverse_proxy/templates/http.conf.j2

@@ -15,8 +15,8 @@ server {
     # Redirect to HTTPS all non-ACME requests
     location / {
       if ($request_method = POST) {
-        return 307 https://$server_name$request_uri;
+        return 307 https://$host$request_uri;
       }
-      return 301 https://$server_name$request_uri;
+      return 301 https://$host$request_uri;
     }
 }