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

contrib/terraform/gcp/generate-inventory.sh

@@ -50,13 +50,13 @@ for name in "${WORKER_NAMES[@]}"; do
 done
 
 echo ""
-echo "[kube-master]"
+echo "[kube_control_plane]"
 for name in "${MASTER_NAMES[@]}"; do
   echo "${name}"
 done
 
 echo ""
-echo "[kube-master:vars]"
+echo "[kube_control_plane:vars]"
 echo "supplementary_addresses_in_ssl_keys = [ '${API_LB}' ]" # Add LB address to API server certificate
 echo ""
 echo "[etcd]"
@@ -72,5 +72,5 @@ done
 
 echo ""
 echo "[k8s-cluster:children]"
-echo "kube-master"
+echo "kube_control_plane"
 echo "kube-node"