Auto-add IPv6DualStack featureGate

When enable_dual_stack_networks is set, we need to make sure
IPv6DualStack=true is set too, otherwise we end up with
a broken cluster.

roles/kubernetes/preinstall/tasks/0040-set_facts.yml

@@ -176,3 +176,10 @@
   set_fact:
     kubelet_flexvolumes_plugins_dir: /var/lib/kubelet/volumeplugins
   when: not usr.stat.writeable
+
+- name: Ensure IPv6DualStack featureGate is set when enable_dual_stack_networks is true
+  set_fact:
+    kube_feature_gates: "{{ kube_feature_gates + [ 'IPv6DualStack=true' ] }}"
+  when:
+    - enable_dual_stack_networks
+    - not 'IPv6DualStack=true' in kube_feature_gates