Purge /root/.kube/config when migrating to kubeadm (#3566)

roles/kubernetes/master/tasks/kubeadm-cleanup-old-certs.yml

@@ -10,3 +10,8 @@
     path: "{{ item.path }}"
     state: absent
   with_items: "{{ files_to_purge_for_kubeadm.files }}"
+
+- name: kubeadm | Purge old kubeconfig
+  file:
+    path: /root/.kube/config
+    state: absent