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/set_cluster_health.yml

@@ -0,0 +1,7 @@
+---
+- name: Configure | Check if cluster is healthy
+  shell: "etcdctl --peers={{ etcd_access_addresses }} cluster-health | grep -q 'cluster is healthy'"
+  register: etcd_cluster_is_healthy
+  ignore_errors: true
+  changed_when: false
+  when: is_etcd_master