Added a description for SPARK_SSH_FOREGROUND variable

docs/spark-standalone.md

@@ -62,7 +62,7 @@ Finally, the following configuration options can be passed to the master and wor
 
 # Cluster Launch Scripts
 
-To launch a Spark standalone cluster with the launch scripts, you need to create a file called `conf/slaves` in your Spark directory, which should contain the hostnames of all the machines where you would like to start Spark workers, one per line. If conf/slaves does not exist, the launch scripts use a list which contains single hostname `localhost`. This can be used for testing. The master machine must be able to access each of the slave machines via password-less `ssh` (using a private key).
+To launch a Spark standalone cluster with the launch scripts, you need to create a file called `conf/slaves` in your Spark directory, which should contain the hostnames of all the machines where you would like to start Spark workers, one per line. If `conf/slaves` does not exist, the launch scripts use a list which contains single hostname `localhost`. This can be used for testing. The master machine must be able to access each of the slave machines via `ssh`. By default, `ssh` is executed in the background for parallel execution for each slave machine. If you would like to use password authentication instead of password-less(using a private key) for `ssh`, `ssh` does not work well in the background. To avoid this, you can set a environment variable `SPARK_SSH_FOREGROUND` to `yes` or `y` to execute `ssh` in the foreground.
 
 Once you've set up this file, you can launch or stop your cluster with the following shell scripts, based on Hadoop's deploy scripts, and available in `SPARK_HOME/bin`: