Running etcd in Docker reduces the number of individual file
downloads and services running on the host.
Note: etcd container v3.0.1 moves bindir to /usr/local/bin
Fixes: #298
* Set ETCD_INITIAL_CLUSTER_STATE from `new` to `existing`,
because parameter `new` makes sense only on cluster assembly
stage.
* If cluster exists and current node is not a part
of the cluster, add it with command `etcdctl add member name url`.
Closes kubespray/kargo/#270