support flannel dual stack (#9564)

roles/network_plugin/flannel/templates/cni-flannel.yml.j2

@@ -31,6 +31,11 @@ data:
   net-conf.json: |
     {
       "Network": "{{ kube_pods_subnet }}",
+      "EnableIPv4": true,
+{%   if enable_dual_stack_networks %}
+      "EnableIPv6": true,
+      "IPv6Network": "{{ kube_pods_subnet_ipv6 }}",
+{%   endif %}
       "Backend": {
         "Type": "{{ flannel_backend_type }}"{% if flannel_backend_type == "vxlan" %},
         "VNI": {{ flannel_vxlan_vni }},