Fix etcd install with docker and etcd_kubeadm_enabled (#5777)

- This solves issue #5721 & #5713 (dupes)
  - Provide a cleaner default usage pattern for the download role
    around etcd that supports 'host' and 'docker' properly
  - Extract the 'etcdctl' as a separate task install piece and reuse it where
    appropriate
  - Update the kubeadm-etcd task to reflect the above change