Replace kube-master with kube_control_plane (#7256)

This replaces kube-master with kube_control_plane because of [1]:

  The Kubernetes project is moving away from wording that is
  considered offensive. A new working group WG Naming was created
  to track this work, and the word "master" was declared as offensive.
  A proposal was formalized for replacing the word "master" with
  "control plane". This means it should be removed from source code,
  documentation, and user-facing configuration from Kubernetes and
  its sub-projects.

NOTE: The reason why this changes it to kube_control_plane not
      kube-control-plane is for valid group names on ansible.

[1]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-cluster-lifecycle/kubeadm/2067-rename-master-label-taint/README.md#motivation

roles/kubernetes-apps/ansible/tasks/cleanup_dns.yml

@@ -6,7 +6,7 @@
   ignore_errors: true
   when:
     - dns_mode in ['coredns', 'coredns_dual']
-    - inventory_hostname == groups['kube-master'][0]
+    - inventory_hostname == groups['kube_control_plane'][0]
 
 - name: Kubernetes Apps | Delete kubeadm CoreDNS
   kube:
@@ -17,7 +17,7 @@
     state: absent
   when:
     - dns_mode in ['coredns', 'coredns_dual']
-    - inventory_hostname == groups['kube-master'][0]
+    - inventory_hostname == groups['kube_control_plane'][0]
     - createdby_annotation.stdout != 'kubespray'
 
 - name: Kubernetes Apps | Delete kubeadm Kube-DNS service
@@ -29,4 +29,4 @@
     state: absent
   when:
     - dns_mode in ['coredns', 'coredns_dual']
-    - inventory_hostname == groups['kube-master'][0]
+    - inventory_hostname == groups['kube_control_plane'][0]