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

tests/templates/inventory-aws.j2

@@ -2,7 +2,7 @@ node1 ansible_ssh_host={{ec2.instances[0].public_ip}} ansible_ssh_user={{ssh_use
 node2 ansible_ssh_host={{ec2.instances[1].public_ip}} ansible_ssh_user={{ssh_user}}
 node3 ansible_ssh_host={{ec2.instances[2].public_ip}} ansible_ssh_user={{ssh_user}}
 
-[kube-master]
+[kube_control_plane]
 node1
 node2
 
@@ -21,12 +21,12 @@ node2
 
 [k8s-cluster:children]
 kube-node
-kube-master
+kube_control_plane
 calico-rr
 
 [calico-rr]
 
-[broken_kube-master]
+[broken_kube_control_plane]
 node2
 
 [broken_etcd]