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-do.j2

@@ -3,7 +3,7 @@
 {% endfor %}
 
 {% if mode is defined and mode == "separate" %}
-[kube-master]
+[kube_control_plane]
 {{droplets.results[0].droplet.name}}
 
 [kube-node]
@@ -15,7 +15,7 @@
 [vault]
 {{droplets.results[2].droplet.name}}
 {% elif mode is defined and mode == "ha" %}
-[kube-master]
+[kube_control_plane]
 {{droplets.results[0].droplet.name}}
 {{droplets.results[1].droplet.name}}
 
@@ -30,13 +30,13 @@
 {{droplets.results[1].droplet.name}}
 {{droplets.results[2].droplet.name}}
 
-[broken_kube-master]
+[broken_kube_control_plane]
 {{droplets.results[1].droplet.name}}
 
 [broken_etcd]
 {{droplets.results[2].droplet.name}}
 {% else %}
-[kube-master]
+[kube_control_plane]
 {{droplets.results[0].droplet.name}}
 
 [kube-node]
@@ -53,5 +53,5 @@
 
 [k8s-cluster:children]
 kube-node
-kube-master
+kube_control_plane
 calico-rr