refact ip stack (#11953)

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

@@ -30,12 +30,14 @@ data:
     }
   net-conf.json: |
     {
+{% if ipv4_stack %}
       "Network": "{{ kube_pods_subnet }}",
       "EnableIPv4": true,
-{%   if enable_dual_stack_networks %}
+{% endif %}
+{% if ipv6_stack %}
       "EnableIPv6": true,
       "IPv6Network": "{{ kube_pods_subnet_ipv6 }}",
-{%   endif %}
+{% endif %}
       "Backend": {
         "Type": "{{ flannel_backend_type }}"{% if flannel_backend_type == "vxlan" %},
         "VNI": {{ flannel_vxlan_vni }},