[SPARK-13414][MESOS] Allow multiple dispatchers to be launched.

## What changes were proposed in this pull request?

Users might want to start multiple mesos dispatchers, as each dispatcher can potentially be part of different roles and used for multi-tenancy.

To allow multiple Mesos dispatchers to be launched, we need to be able to specify a instance number when starting the dispatcher daemon.

## How was the this patch tested?

Manual testing

Author: Timothy Chen <tnachen@gmail.com>

Closes #11281 from tnachen/multiple_cluster_dispatchers.

sbin/start-mesos-dispatcher.sh

@@ -37,5 +37,8 @@ if [ "$SPARK_MESOS_DISPATCHER_HOST" = "" ]; then
   SPARK_MESOS_DISPATCHER_HOST=`hostname`
 fi
 
+if [ "$SPARK_MESOS_DISPATCHER_NUM" = "" ]; then
+  SPARK_MESOS_DISPATCHER_NUM=1
+fi
 
-"${SPARK_HOME}/sbin"/spark-daemon.sh start org.apache.spark.deploy.mesos.MesosClusterDispatcher 1 --host $SPARK_MESOS_DISPATCHER_HOST --port $SPARK_MESOS_DISPATCHER_PORT "$@"
+"${SPARK_HOME}/sbin"/spark-daemon.sh start org.apache.spark.deploy.mesos.MesosClusterDispatcher $SPARK_MESOS_DISPATCHER_NUM --host $SPARK_MESOS_DISPATCHER_HOST --port $SPARK_MESOS_DISPATCHER_PORT "$@"

sbin/stop-mesos-dispatcher.sh

@@ -24,5 +24,10 @@ fi
 
 . "${SPARK_HOME}/sbin/spark-config.sh"
 
-"${SPARK_HOME}/sbin"/spark-daemon.sh stop org.apache.spark.deploy.mesos.MesosClusterDispatcher 1
+if [ "$SPARK_MESOS_DISPATCHER_NUM" = "" ]; then
+  SPARK_MESOS_DISPATCHER_NUM=1
+fi
+
+"${SPARK_HOME}/sbin"/spark-daemon.sh stop org.apache.spark.deploy.mesos.MesosClusterDispatcher \
+    $SPARK_MESOS_DISPATCHER_NUM