[SPARK-4325] Use pssh.

Replace some bash-isms with pssh to neatly parallelize cluster operations.

Also, decrease questionably high sleep times.
On EC2 there are two hostnames, one internal (of the form ip-127-1-1-1.ec2.internal) and one external (of the form ec2-54-227-51-123.compute-1.amazonaws.com) -- We typically pass in the latter in `$MASTERS` and `hostname` usually returns the former.

Even though we try to only use the external hostname in all our configs, it is better practice to approve keys for both hostnames