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

docs/aws.md

@@ -35,11 +35,11 @@ This will produce an inventory that is passed into Ansible that looks like the f
   ],
   "k8s-cluster": {
     "children": [
-      "kube-master",
+      "kube_control_plane",
       "kube-node"
     ]
   },
-  "kube-master": [
+  "kube_control_plane": [
     "ip-172-31-3-xxx.us-east-2.compute.internal"
   ],
   "kube-node": [
@@ -51,7 +51,7 @@ This will produce an inventory that is passed into Ansible that looks like the f
 Guide:
 
 - Create instances in AWS as needed.
-- Either during or after creation, add tags to the instances with a key of `kubespray-role` and a value of `kube-master`, `etcd`, or `kube-node`. You can also share roles like `kube-master, etcd`
+- Either during or after creation, add tags to the instances with a key of `kubespray-role` and a value of `kube_control_plane`, `etcd`, or `kube-node`. You can also share roles like `kube_control_plane, etcd`
 - Copy the `kubespray-aws-inventory.py` script from `kubespray/contrib/aws_inventory` to the `kubespray/inventory` directory.
 - Set the following AWS credentials and info as environment variables in your terminal: