allow non existing etcd group (#6797)

When using kubeadm managed etcd, configuring an etcd group can now
be skipped.

roles/kubernetes/preinstall/tasks/0020-verify-settings.yml

@@ -1,14 +1,22 @@
 ---
-- name: Stop if either kube-master, kube-node or etcd is empty
+- name: Stop if either kube-master or kube-node group is empty
   assert:
-    that: groups.get('{{ item }}')
+    that: "groups.get('{{ item }}')"
   with_items:
     - kube-master
     - kube-node
-    - etcd
   run_once: true
   when: not ignore_assert_errors
 
+- name: Stop if etcd group is empty in external etcd mode
+  assert:
+    that: groups.get('etcd')
+    fail_msg: "Group 'etcd' cannot be empty in external etcd mode"
+  run_once: true
+  when:
+    - not ignore_assert_errors
+    - not etcd_kubeadm_enabled
+
 - name: Stop if non systemd OS type
   assert:
     that: ansible_service_mgr == "systemd"
@@ -61,6 +69,7 @@
     that: groups.etcd|length is not divisibleby 2
   when:
     - not ignore_assert_errors
+    - groups.get('etcd')
     - inventory_hostname in groups['etcd']
 
 - name: Stop if memory is too small for masters