WIP: feat(controller): Use etcd directly for scheduling

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.
@johanneswuerbach Done: https://s3-us-west-2.amazonaws.com/opdemand/fleetctl-v0.6.2