[SPARK-3584] sbin/slaves doesn't work when we use password authentication for SSH

what about -

To launch a Spark standalone cluster with the launch scripts, you should create a file called `conf/slaves` in your Spark directory, which must contain the hostnames of all the machines where you intend to start Spark workers, one per line. If `conf/slaves` does not exist, the launch scripts defaults to a single machine (`localhost`), which is useful for testing. Note, the master machine accesses each of the worker machines via `ssh`. By default, `ssh` is run in parallel and requires password-less (using a private key) access to be setup. If you do not have a password-less setup, you can set the environment variable `SPARK_SSH_FOREGROUND` and serially provide a password for each worker.