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.
I would like to change this to:
```
Usage: deis clusters:create <id> <domain> (--hosts=<hosts> | --etcd-discovery-url=<etcd-discovery-url>) --auth=<auth> [options]
```
So either `hosts` or `etcd-discovery-url` is required, but then flake8 is complaining about the long line. Is there a way I can split this into multiple lines @bacongobbler?