[YARN][SPARK-2606]:In some cases,the spark UI pages display incorrect

The issue is caused by #1112 .

Author: GuoQiang Li <witgo@qq.com>

Closes #1501 from witgo/webui_style and squashes the following commits:

4b34998 [GuoQiang Li] In some cases, pages display incorrect in WebUI

core/src/main/scala/org/apache/spark/ui/UIUtils.scala

@@ -149,7 +149,7 @@ private[spark] object UIUtils extends Logging {
 
   def prependBaseUri(basePath: String = "", resource: String = "") = uiRoot + basePath + resource
 
-  val commonHeaderNodes = {
+  def commonHeaderNodes = {
     <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
     <link rel="stylesheet" href={prependBaseUri("/static/bootstrap.min.css")}
           type="text/css" />