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.
@philips yes, this is just the simplest thing I can do right now to get this patch "done" and in. I'm working on major improvements to this in a subsequent branch.