Fix readOnly flag in kubeadm-config.v1beta1.yaml.j2

In v1beta1 of `ClusterConfiguration` the extraVolumes `writable` field was changed to `readOnly` and its boolean value must be negated.

Also, the json field for `useHyperKubeImage` was incorrectly capitalized.

roles/kubernetes/master/defaults/main/main.yml

@@ -143,7 +143,7 @@ kube_kubeadm_scheduler_extra_args: {}
 #  - name: name
 #    hostPath: /host/path
 #    mountPath: /mount/path
-#    writable: false
+#    readOnly: true
 apiserver_extra_volumes: {}
 controller_manager_extra_volumes: {}
 scheduler_extra_volumes: {}