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

Vagrantfile

@@ -253,9 +253,9 @@ Vagrant.configure("2") do |config|
           #ansible.tags = ['download']
           ansible.groups = {
             "etcd" => ["#{$instance_name_prefix}-[1:#{$etcd_instances}]"],
-            "kube-master" => ["#{$instance_name_prefix}-[1:#{$kube_master_instances}]"],
+            "kube_control_plane" => ["#{$instance_name_prefix}-[1:#{$kube_master_instances}]"],
             "kube-node" => ["#{$instance_name_prefix}-[1:#{$kube_node_instances}]"],
-            "k8s-cluster:children" => ["kube-master", "kube-node"],
+            "k8s-cluster:children" => ["kube_control_plane", "kube-node"],
           }
         end
       end