Warn when user has no etcd group member nodes

This will be removed from support in a future release

playbooks/common/openshift-cluster/config.yml

@@ -22,6 +22,12 @@
       - docker_image_availability
       - docker_storage
 
+- hosts: localhost
+  tasks:
+  - fail:
+      msg: No etcd hosts defined. Running an all-in-one master is deprecated and will no longer be supported in a future upgrade.
+    when: groups.oo_etcd_to_config | default([]) | length == 0 and not openshift_master_unsupported_all_in_one | default(False)
+
 - include: initialize_oo_option_facts.yml
   tags:
   - always