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_facts.yml

@@ -0,0 +1,16 @@
+---
+- set_fact: etcd_access_address="{{ access_ip | default(ip | default(ansible_default_ipv4['address'])) }}"
+- set_fact: etcd_peer_url="http://{{ etcd_access_address }}:2380"
+- set_fact: etcd_client_url="http://{{ etcd_access_address }}:2379"
+- set_fact:
+    etcd_access_addresses: |-
+      {% for item in groups['etcd'] -%}
+        http://{{ hostvars[item].etcd_access_address }}:2379{% if not loop.last %},{% endif %}
+      {%- endfor %}
+- set_fact:
+    etcd_member_name: |-
+      {% for host in groups['etcd'] %}
+      {%   if inventory_hostname == host %}{{"etcd"+loop.index|string }}{% endif %}
+      {% endfor %}
+- set_fact:
+    is_etcd_master: "{{ inventory_hostname in groups['etcd'] }}"