[SPARK-5771][UI][hotfix] Change Requested Cores into * if default cores is not set

cc andrewor14, srowen.

Author: jerryshao <saisai.shao@intel.com>

Closes #4800 from jerryshao/SPARK-5771 and squashes the following commits:

a2483c2 [jerryshao] Change the UI of Requested Cores into * if default cores is not set

core/src/main/scala/org/apache/spark/deploy/master/ui/MasterPage.scala

@@ -182,7 +182,7 @@ private[spark] class MasterPage(parent: MasterWebUI) extends WebUIPage("") {
         }
       }
       <td>
-        {app.requestedCores}
+        {if (app.requestedCores == Int.MaxValue) "*" else app.requestedCores}
       </td>
       <td sorttable_customkey={app.desc.memoryPerSlave.toString}>
         {Utils.megabytesToString(app.desc.memoryPerSlave)}