Support completion for --cluster and --user

With this commit, kubectl is supported completion for `--cluster` and `--user`.

```
$ kubectl --user=<tab>
--user=minikube  --user=user01

$ kubectl --cluster=<tab>
--cluster=cluster01  --cluster=minikube
```

pkg/kubectl/cmd/cmd.go

@@ -76,10 +76,26 @@ __kubectl_get_namespaces()
     fi
 }
 
-__kubectl_get_contexts()
+__kubectl_config_get_contexts()
+{
+    __kubectl_parse_config "contexts"
+}
+
+__kubectl_config_get_clusters()
+{
+    __kubectl_parse_config "clusters"
+}
+
+__kubectl_config_get_users()
+{
+    __kubectl_parse_config "users"
+}
+
+# $1 has to be "contexts", "clusters" or "users"
+__kubectl_config_get()
 {
     local template kubectl_out
-    template="{{ range .contexts  }}{{ .name }} {{ end }}"
+    template="{{ range .$1  }}{{ .name }} {{ end }}"
     if kubectl_out=$(kubectl config $(__kubectl_override_flags) -o template --template="${template}" view 2>/dev/null); then
         COMPREPLY=( $( compgen -W "${kubectl_out[*]}" -- "$cur" ) )
     fi
@@ -171,7 +187,7 @@ __custom_func() {
             return
             ;;
         kubectl_config_use-context)
-            __kubectl_get_contexts
+            __kubectl_config_get_contexts
             return
             ;;
         *)
@@ -224,6 +240,15 @@ __custom_func() {
     `
 )
 
+var (
+	bash_completion_flags = map[string]string{
+		"namespace": "__kubectl_get_namespaces",
+		"context":   "__kubectl_config_get_contexts",
+		"cluster":   "__kubectl_config_get_clusters",
+		"user":      "__kubectl_config_get_users",
+	}
+)
+
 // NewKubectlCommand creates the `kubectl` command and its nested children.
 func NewKubectlCommand(f cmdutil.Factory, in io.Reader, out, err io.Writer) *cobra.Command {
 	// Parent command to which all subcommands are added.
@@ -330,24 +355,16 @@ func NewKubectlCommand(f cmdutil.Factory, in io.Reader, out, err io.Writer) *cob
 	}
 	templates.ActsAsRootCommand(cmds, filters, groups...)
 
-	if cmds.Flag("namespace") != nil {
-		if cmds.Flag("namespace").Annotations == nil {
-			cmds.Flag("namespace").Annotations = map[string][]string{}
+	for name, completion := range bash_completion_flags {
+		if cmds.Flag(name) != nil {
+			if cmds.Flag(name).Annotations == nil {
+				cmds.Flag(name).Annotations = map[string][]string{}
+			}
+			cmds.Flag(name).Annotations[cobra.BashCompCustom] = append(
+				cmds.Flag(name).Annotations[cobra.BashCompCustom],
+				completion,
+			)
 		}
-		cmds.Flag("namespace").Annotations[cobra.BashCompCustom] = append(
-			cmds.Flag("namespace").Annotations[cobra.BashCompCustom],
-			"__kubectl_get_namespaces",
-		)
-	}
-
-	if cmds.Flag("context") != nil {
-		if cmds.Flag("context").Annotations == nil {
-			cmds.Flag("context").Annotations = map[string][]string{}
-		}
-		cmds.Flag("context").Annotations[cobra.BashCompCustom] = append(
-			cmds.Flag("context").Annotations[cobra.BashCompCustom],
-			"__kubectl_get_contexts",
-		)
 	}
 
 	cmds.AddCommand(cmdconfig.NewCmdConfig(clientcmd.NewDefaultPathOptions(), out, err))