[SPARK-18114][MESOS] Fix mesos cluster scheduler generage command option error

## What changes were proposed in this pull request?

Enclose --conf option value with "" to support multi value configs like spark.driver.extraJavaOptions, without "", driver will fail to start.
## How was this patch tested?

Jenkins Tests.

Test in our production environment, also unit tests, It is a very small change.

Author: Wang Lei <lei.wang@kongming-inc.com>

Closes #15643 from LeightonWong/messos-cluster.

mesos/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosClusterScheduler.scala

@@ -482,7 +482,7 @@ private[spark] class MesosClusterScheduler(
       .filter { case (key, _) => !replicatedOptionsBlacklist.contains(key) }
       .toMap
     (defaultConf ++ driverConf).foreach { case (key, value) =>
-      options ++= Seq("--conf", s"$key=${shellEscape(value)}") }
+      options ++= Seq("--conf", s""""$key=${shellEscape(value)}"""".stripMargin) }
 
     options
   }