Rework inventory all by real groups' vars

* Leave all.yml to keep only optional vars
* Store groups' specific vars by existing group names
* Fix optional vars casted as mandatory (add default())
* Fix missing defaults for an optional IP var
* Relink group_vars for terraform to reflect changes

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>

roles/adduser/defaults/main.yml

@@ -1,4 +1,6 @@
 ---
+kube_cert_group: kube-cert
+
 addusers:
   etcd:
     name: etcd