Ensuring right apis are called as per jetty version in branch2 (trunk has different jetty)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/webapp/WebApps.java

@@ -402,7 +402,7 @@ public class WebApps {
       HttpServer2 httpServer = webApp.httpServer();
       if (ui2Context != null) {
         addFiltersForNewContext(ui2Context);
-        httpServer.addHandlerAtFront(ui2Context);
+        httpServer.addContext(ui2Context, true);
       }
       try {
         httpServer.start();