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-gce.j2

@@ -9,7 +9,7 @@
 {{node3}} ansible_ssh_host={{gce.instance_data[2].public_ip}}
 {% endif %}
 {% if mode is defined and mode in ["separate", "separate-scale"] %}
-[kube-master]
+[kube_control_plane]
 {{node1}}
 
 [kube-node]
@@ -21,7 +21,7 @@
 [vault]
 {{node3}}
 {% elif mode is defined and mode in ["ha", "ha-scale"] %}
-[kube-master]
+[kube_control_plane]
 {{node1}}
 {{node2}}
 
@@ -38,14 +38,14 @@
 {{node2}}
 {{node3}}
 
-[broken_kube-master]
+[broken_kube_control_plane]
 {{node2}}
 
 [etcd]
 {{node2}}
 {{node3}}
 {% elif mode == "default" %}
-[kube-master]
+[kube_control_plane]
 {{node1}}
 
 [kube-node]
@@ -57,7 +57,7 @@
 [vault]
 {{node1}}
 {% elif mode == "aio" %}
-[kube-master]
+[kube_control_plane]
 {{node1}}
 
 [kube-node]
@@ -72,7 +72,7 @@
 
 [k8s-cluster:children]
 kube-node
-kube-master
+kube_control_plane
 calico-rr
 
 [calico-rr]