[SPARK-874] adding a --wait flag

This PR adds a --wait flag to the `./sbin/stop-all.sh` script.

Author: jbencook <jbenjamincook@gmail.com>

Closes #3567 from jbencook/master and squashes the following commits:

d05c5bb [jbencook] [SPARK-874] adding a --wait flag

sbin/stop-all.sh

@@ -30,3 +30,20 @@ sbin="`cd "$sbin"; pwd`"
 # Stop the slaves, then the master
 "$sbin"/stop-slaves.sh
 "$sbin"/stop-master.sh
+
+if [ "$1" == "--wait" ]
+then
+  printf "Waiting for workers to shut down..."
+  while true
+  do
+    running=`$sbin/slaves.sh ps -ef | grep -v grep | grep deploy.worker.Worker`
+    if [ -z "$running" ]
+    then
+      printf "\nAll workers successfully shut down.\n"
+      break
+    else
+      printf "."
+      sleep 10
+    fi
+  done
+fi