[SPARK-3398] [SPARK-4325] [EC2] Use EC2 status checks.

This PR re-introduces [0e648bc](https://github.com/apache/spark/commit/0e648bc2bedcbeb55fce5efac04f6dbad9f063b4) from PR #2339, which somehow never made it into the codebase.

Additionally, it removes a now-unnecessary linear backoff on the SSH checks since we are blocking on EC2 status checks before testing SSH.
Good point. I'll fix that.