Configure Etcd container_manager explicitly (kubernetes-sigs#8521)

* Configure Etcd container_manager explicitly

* Add explanation for the Etcd container_manager variable

* Remove redundant space in etcd vars

inventory/sample/group_vars/etcd.yml

@@ -18,6 +18,13 @@
 # This affects ETCD_PEER_CLIENT_CERT_AUTH variable
 # etcd_peer_client_auth: true
 
+## Container runtime
+## docker for docker, crio for cri-o and containerd for containerd.
+## If this is not set, container manager will be inherited from the Kubespray defaults
+## and not from k8s_cluster/k8s-cluster.yml, which might not be what you want.
+## Also this makes possible to use different container manager for etcd nodes.
+# container_manager: containerd
+
 ## Settings for etcd deployment type
 # Set this to docker if you are using container_manager: docker
 etcd_deployment_type: host