ls consistency: add tctl kube ls command

```
Cluster     Labels Version
----------- ------ -------------
minikube           8.0.0-alpha.1
honkcluster        8.0.3
```

```yaml
kind: kube_service
metadata:
  expires: "2021-12-16T16:07:14.898611765Z"
  id: 1639670234899200604
  name: 7b3ca8d8-710c-4305-aa29-e73628ac572c
spec:
  addr: 127.0.0.1:3027
  hostname: ""
  kube_clusters:
  - name: minikube
  rotation:
    current_id: ""
    last_rotated: "0001-01-01T00:00:00Z"
    schedule:
      standby: "0001-01-01T00:00:00Z"
      update_clients: "0001-01-01T00:00:00Z"
      update_servers: "0001-01-01T00:00:00Z"
    started: "0001-01-01T00:00:00Z"
  version: 8.0.0-alpha.1
version: v2
---
kind: kube_service
metadata:
  expires: "2021-12-16T16:10:13.068855399Z"
  id: 1639670413069866294
  name: 9153a5a9-e85e-4972-8c50-6fa8923282b2
spec:
  addr: remote.kube.proxy.teleport.cluster.local
  hostname: ""
  kube_clusters:
  - name: honkcluster
  rotation:
    current_id: ""
    last_rotated: "0001-01-01T00:00:00Z"
    schedule:
      standby: "0001-01-01T00:00:00Z"
      update_clients: "0001-01-01T00:00:00Z"
      update_servers: "0001-01-01T00:00:00Z"
    started: "0001-01-01T00:00:00Z"
  version: 8.0.3
version: v2
```yaml

tool/tctl/common/collection.go

@@ -751,3 +751,27 @@ func (c *tokenCollection) writeText(w io.Writer) error {
 	}
 	return nil
 }
+
+type kubeServerCollection struct {
+	servers []types.Server
+}
+
+func (c *kubeServerCollection) writeText(w io.Writer) error {
+	t := asciitable.MakeTable([]string{"Cluster", "Labels", "Version"})
+	for _, server := range c.servers {
+		kubes := server.GetKubernetesClusters()
+		for _, kube := range kubes {
+			t.AddRow([]string{
+				kube.Name,
+				types.LabelsAsString(kube.StaticLabels, kube.DynamicLabels),
+				server.GetTeleportVersion(),
+			})
+		}
+	}
+	_, err := t.AsBuffer().WriteTo(w)
+	return trace.Wrap(err)
+}
+
+func (c *kubeServerCollection) writeYAML(w io.Writer) error {
+	return utils.WriteYAML(w, c.servers)
+}

tool/tctl/common/kube_command.go

@@ -0,0 +1,82 @@
+/*
+Copyright 2021 Gravitational, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package common
+
+import (
+	"context"
+	"os"
+
+	"github.com/gravitational/kingpin"
+	"github.com/gravitational/trace"
+
+	"github.com/gravitational/teleport"
+	"github.com/gravitational/teleport/lib/auth"
+	"github.com/gravitational/teleport/lib/service"
+)
+
+// KubeCommand implements "tctl kube" group of commands.
+type KubeCommand struct {
+	config *service.Config
+
+	// format is the output format (text or yaml)
+	format string
+
+	// kubeList implements the "tctl kube ls" subcommand.
+	kubeList *kingpin.CmdClause
+}
+
+// Initialize allows KubeCommand to plug itself into the CLI parser
+func (c *KubeCommand) Initialize(app *kingpin.Application, config *service.Config) {
+	c.config = config
+
+	kube := app.Command("kube", "Operate on registered kubernetes clusters.")
+	c.kubeList = kube.Command("ls", "List all kubernetes clusters registered with the cluster.")
+	c.kubeList.Flag("format", "Output format, 'text', or 'yaml'").Default("text").StringVar(&c.format)
+}
+
+// TryRun attempts to run subcommands like "kube ls".
+func (c *KubeCommand) TryRun(cmd string, client auth.ClientI) (match bool, err error) {
+	switch cmd {
+	case c.kubeList.FullCommand():
+		err = c.ListKube(client)
+	default:
+		return false, nil
+	}
+	return true, trace.Wrap(err)
+}
+
+// ListKube prints the list of kube clusters that have recently sent heartbeats
+// to the cluster.
+func (c *KubeCommand) ListKube(client auth.ClientI) error {
+	kubes, err := client.GetKubeServices(context.TODO())
+	if err != nil {
+		return trace.Wrap(err)
+	}
+	coll := &kubeServerCollection{servers: kubes}
+	switch c.format {
+	case teleport.Text:
+		err = coll.writeText(os.Stdout)
+	case teleport.YAML:
+		err = coll.writeYAML(os.Stdout)
+	default:
+		return trace.BadParameter("unknown format %q", c.format)
+	}
+	if err != nil {
+		return trace.Wrap(err)
+	}
+	return nil
+}

tool/tctl/main.go

@@ -32,6 +32,7 @@ func main() {
 		&common.AccessRequestCommand{},
 		&common.AppsCommand{},
 		&common.DBCommand{},
+		&common.KubeCommand{},
 		&common.AccessCommand{},
 		&common.LockCommand{},
 		&common.BotsCommand{},