Replace KUBE_MASTERS with KUBE_CONTROL_HOSTS (kubernetes-sigs#7257)

This replaces KUBE_MASTERS with KUBE_CONTROL_HOSTS 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.
```

[1]: https://github.com/kubernetes/enhancements/blob/master/keps/sig-cluster-lifecycle/kubeadm/2067-rename-master-label-taint/README.md#motivation

contrib/inventory_builder/inventory.py

@@ -63,7 +63,9 @@ def get_var_as_bool(name, default):
 
 
 CONFIG_FILE = os.environ.get("CONFIG_FILE", "./inventory/sample/hosts.yaml")
-KUBE_MASTERS = int(os.environ.get("KUBE_MASTERS", 2))
+# Remove the reference of KUBE_MASTERS after some deprecation cycles.
+KUBE_CONTROL_HOSTS = int(os.environ.get("KUBE_CONTROL_HOSTS",
+                         os.environ.get("KUBE_MASTERS", 2)))
 # Reconfigures cluster distribution at scale
 SCALE_THRESHOLD = int(os.environ.get("SCALE_THRESHOLD", 50))
 MASSIVE_SCALE_THRESHOLD = int(os.environ.get("MASSIVE_SCALE_THRESHOLD", 200))
@@ -102,10 +104,11 @@ def __init__(self, changed_hosts=None, config_file=None):
             etcd_hosts_count = 3 if len(self.hosts.keys()) >= 3 else 1
             self.set_etcd(list(self.hosts.keys())[:etcd_hosts_count])
             if len(self.hosts) >= SCALE_THRESHOLD:
-                self.set_kube_master(list(self.hosts.keys())[
-                    etcd_hosts_count:(etcd_hosts_count + KUBE_MASTERS)])
+                self.set_kube_control_plane(list(self.hosts.keys())[
+                    etcd_hosts_count:(etcd_hosts_count + KUBE_CONTROL_HOSTS)])
             else:
-                self.set_kube_master(list(self.hosts.keys())[:KUBE_MASTERS])
+                self.set_kube_control_plane(
+                  list(self.hosts.keys())[:KUBE_CONTROL_HOSTS])
             self.set_kube_node(self.hosts.keys())
             if len(self.hosts) >= SCALE_THRESHOLD:
                 self.set_calico_rr(list(self.hosts.keys())[:etcd_hosts_count])
@@ -294,7 +297,7 @@ def add_host_to_group(self, group, host, opts=""):
             else:
                 self.yaml_config['all']['children'][group]['hosts'][host] = None  # noqa
 
-    def set_kube_master(self, hosts):
+    def set_kube_control_plane(self, hosts):
         for host in hosts:
             self.add_host_to_group('kube-master', host)
 
@@ -402,9 +405,9 @@ def show_help(self):
 DEBUG                   Enable debug printing. Default: True
 CONFIG_FILE             File to write config to Default: ./inventory/sample/hosts.yaml
 HOST_PREFIX             Host prefix for generated hosts. Default: node
-KUBE_MASTERS            Set the number of kube-masters. Default: 2
+KUBE_CONTROL_HOSTS      Set the number of kube-control-planes. Default: 2
 SCALE_THRESHOLD         Separate ETCD role if # of nodes >= 50
-MASSIVE_SCALE_THRESHOLD Separate K8s master and ETCD if # of nodes >= 200
+MASSIVE_SCALE_THRESHOLD Separate K8s control-plane and ETCD if # of nodes >= 200
 '''  # noqa
         print(help_text)
 

contrib/inventory_builder/tests/test_inventory.py

@@ -222,11 +222,11 @@ def test_add_host_to_group(self):
             self.inv.yaml_config['all']['children'][group]['hosts'].get(host),
             None)
 
-    def test_set_kube_master(self):
+    def test_set_kube_control_plane(self):
         group = 'kube-master'
         host = 'node1'
 
-        self.inv.set_kube_master([host])
+        self.inv.set_kube_control_plane([host])
         self.assertIn(
             host, self.inv.yaml_config['all']['children'][group]['hosts'])
 
@@ -275,7 +275,7 @@ def test_scale_scenario_one(self):
 
         self.inv.set_all(hosts)
         self.inv.set_etcd(list(hosts.keys())[0:3])
-        self.inv.set_kube_master(list(hosts.keys())[0:2])
+        self.inv.set_kube_control_plane(list(hosts.keys())[0:2])
         self.inv.set_kube_node(hosts.keys())
         for h in range(3):
             self.assertFalse(
@@ -291,7 +291,7 @@ def test_scale_scenario_two(self):
 
         self.inv.set_all(hosts)
         self.inv.set_etcd(list(hosts.keys())[0:3])
-        self.inv.set_kube_master(list(hosts.keys())[3:5])
+        self.inv.set_kube_control_plane(list(hosts.keys())[3:5])
         self.inv.set_kube_node(hosts.keys())
         for h in range(5):
             self.assertFalse(