Scale-up functionality for etcd cluster

* 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

roles/etcd/tasks/refresh_config.yml

@@ -0,0 +1,6 @@
+---
+- name: Refresh config | Create etcd config file
+  template:
+    src: etcd.j2
+    dest: /etc/etcd.env
+  notify: restart etcd