[SPARK-4325] Use pssh.

Replace some bash-isms with pssh to neatly parallelize cluster operations.

Also, decrease questionably high sleep times.
This is less a performance optimization and more just code trimming. Now that we [check for SSH availability across the cluster in `spark_ec2.py`](https://github.com/apache/spark/blob/b32734e12d5197bad26c080e529edd875604c6fb/ec2/spark_ec2.py#L961), there is no need for the retry logic here.