fix kubelet_flexvolumes_plugins_dir undefined (#6645)

roles/kubernetes-apps/external_cloud_controller/openstack/templates/external-openstack-cloud-controller-manager-ds.yml.j2

@@ -59,8 +59,10 @@ spec:
               name: openstack-cacert
               readOnly: true
 {% endif %}
+{% if kubelet_flexvolumes_plugins_dir is defined %}
             - mountPath: /usr/libexec/kubernetes/kubelet-plugins/volume/exec
               name: flexvolume-dir
+{% endif %}
           resources:
             requests:
               cpu: 200m
@@ -69,10 +71,12 @@ spec:
               value: /etc/config/cloud.conf
       hostNetwork: true
       volumes:
+{% if kubelet_flexvolumes_plugins_dir is defined %}
       - hostPath:
           path: "{{ kubelet_flexvolumes_plugins_dir }}"
           type: DirectoryOrCreate
         name: flexvolume-dir
+{% endif %}
       - hostPath:
           path: /etc/kubernetes/pki
           type: DirectoryOrCreate