added serverMetadata option for layer group, close #182 CDB-1940

config/environments/staging.js.example

@@ -116,6 +116,12 @@ var config = {
     // X-Tiler-Profile header containing elapsed timing for various 
     // steps taken for producing the response.
     ,useProfiler:true
+    ,serverMetadata: {
+      cdn_url: {
+        http: 'api.cartocdn.com',
+        https: 'cartocdn.global.ssl.fastly.net'
+      }
+    }
 };
 
 module.exports = config;