Bug 1060312 - Improve the API back-end prefs in the sample local config

It's now easier to switch between the API back-ends and harder to make
typos. The default backend has also been changed to production (instead
of using the same host), since local.conf.js is now optional, so if
someone just wants to use the same host as the UI for the back-end, then
they can just not create local.conf.js in the first place. But for those
who _do_ want to use production (eg those using web-server.js), initial
setup is now a single |cp sample.local.conf.js local.conf.js| rather
than also having to edit the file afterwards.

ui/js/config/sample.local.conf.js

@@ -4,16 +4,15 @@
 
 'use strict';
 
-/* window.thServiceDomain holds a reference to a backend service
- * for result data. This can be one of:
- *
- * -  http://local.treeherder.mozilla.org (local vagrant)
- * -  http://treeherder-dev.allizom.org (dev)
- * -  https://treeherder.allizom.org (stage)
- * -  https://treeherder.mozilla.org (production) */
+/* window.thServiceDomain holds a reference to a back-end service
+ * used to retrieve job result data. Valid settings are  */
+var production = "https://treeherder.mozilla.org";
+var stage = "https://treeherder.allizom.org";
+var dev = "http://treeherder-dev.allizom.org";
+var vagrant = "";
 
-// By default the service domain is the current host name.
-window.thServiceDomain = "";
+// Set the service
+window.thServiceDomain = production;
 
 //treeherder.config(['$logProvider', 'ThLogConfigProvider',
 //    function($logProvider, ThLogConfigProvider) {