use etcd2 when upgrading unless forced

roles/kubernetes/master/defaults/main.yml

@@ -16,6 +16,9 @@ etcd_cert_dir: "{{ etcd_config_dir }}/ssl"
 # ETCD backend for k8s data
 kube_apiserver_storage_backend: etcd3
 
+# By default, force back to etcd2. Set to true to force etcd3 (experimental!)
+force_etcd3: false
+
 # Limits for kube components
 kube_controller_memory_limit: 512M
 kube_controller_cpu_limit: 250m