[SPARK-7962] [MESOS] Fix master url parsing in rest submission client.

Only parse standalone master url when master url starts with spark://

Author: Timothy Chen <tnachen@gmail.com>

Closes #6517 from tnachen/fix_mesos_client and squashes the following commits:

61a1198 [Timothy Chen] Fix master url parsing in rest submission client.

core/src/main/scala/org/apache/spark/deploy/rest/RestSubmissionClient.scala

@@ -57,7 +57,11 @@ private[spark] class RestSubmissionClient(master: String) extends Logging {
 
   private val supportedMasterPrefixes = Seq("spark://", "mesos://")
 
-  private val masters: Array[String] = Utils.parseStandaloneMasterUrls(master)
+  private val masters: Array[String] = if (master.startsWith("spark://")) {
+    Utils.parseStandaloneMasterUrls(master)
+  } else {
+    Array(master)
+  }
 
   // Set of masters that lost contact with us, used to keep track of
   // whether there are masters still alive for us to communicate with