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

inventory/sample/inventory.ini

@@ -13,7 +13,7 @@
 # [bastion]
 # bastion ansible_host=x.x.x.x ansible_user=some_user
 
-[kube-master]
+[kube_control_plane]
 # node1
 # node2
 # node3
@@ -33,6 +33,6 @@
 [calico-rr]
 
 [k8s-cluster:children]
-kube-master
+kube_control_plane
 kube-node
 calico-rr