WIP: feat(*): Find cluster hosts via etcd discovery url

Discover cluster hosts using a etcd discovery url instead of hardcoding a list of hosts

Partially fixes #1727

@carmstrong we discussed about this a while ago and actually it seems to be relatively easy. Maybe it could be enhanced further by using the last updated host instead of randomly trying one.
You could shorten the arg name and value to `--discovery-url=<url>` perhaps, but continue to call it "etcd discovery url" in the usage description.