roachprod: implement --dry-run and --verbose

Currently, roachprod debugging consists of either trying to match the output to the code, or executing (cloud) commands to observe their effects. There is also some bootstrapping "magic" (e.g., ssh setup) which happens without much of a trace in the logged output. Consequently, even an experienced roachprod user may end up spending considerable time to diagnose an infra. issue.

Both the `--dry-run`, and the `--verbose` mode should make it easier to troubleshoot roachprod issues. By logging the commands, the "magic" usually becomes self-explanatory. Furthermore, `--dry-run` would considerably speed up debugging without having to apply actual infra. changes.

Jira issue: CRDB-25697