Use `-peers` in `etcdctl members` commands

This is still incredibly stupid, but it gets us far enough for v0.5.0-alpha.1. It only uses the first argument to `-peers` without any retries, but the framework is there to introduce the retry logic ASAP. I plan to continue improving this immediately.
Yeah, I was thinking about how this should really be done and over-complicated it. I'll just simplify this down for now.