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/inventory_builder/tests/test_inventory.py

@@ -223,7 +223,7 @@ class TestInventory(unittest.TestCase):
             None)
 
     def test_set_kube_control_plane(self):
-        group = 'kube-master'
+        group = 'kube_control_plane'
         host = 'node1'
 
         self.inv.set_kube_control_plane([host])
@@ -242,7 +242,7 @@ class TestInventory(unittest.TestCase):
 
     def test_set_k8s_cluster(self):
         group = 'k8s-cluster'
-        expected_hosts = ['kube-node', 'kube-master']
+        expected_hosts = ['kube-node', 'kube_control_plane']
 
         self.inv.set_k8s_cluster()
         for host in expected_hosts: