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/aws_inventory/kubespray-aws-inventory.py

@@ -35,7 +35,7 @@ class SearchEC2Tags(object):
     hosts['_meta'] = { 'hostvars': {} }
 
     ##Search ec2 three times to find nodes of each group type. Relies on kubespray-role key/value.
-    for group in ["kube-master", "kube-node", "etcd"]:
+    for group in ["kube_control_plane", "kube-node", "etcd"]:
       hosts[group] = []
       tag_key = "kubespray-role"
       tag_value = ["*"+group+"*"]
@@ -70,7 +70,7 @@ class SearchEC2Tags(object):
         hosts[group].append(dns_name)
         hosts['_meta']['hostvars'][dns_name] = ansible_host
         
-    hosts['k8s-cluster'] = {'children':['kube-master', 'kube-node']}
+    hosts['k8s-cluster'] = {'children':['kube_control_plane', 'kube-node']}
     print(json.dumps(hosts, sort_keys=True, indent=2))
 
 SearchEC2Tags()