cmd/containerboot,kube: consolidate the two kube clients

We had two implemenetations of the kube client, merge them.

containerboot was also using a raw http.Transport, this also has
the side effect of making it use a http.Client

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/containerboot/main.go

@@ -123,7 +123,7 @@ func main() {
 	defer cancel()
 
 	if cfg.InKubernetes && cfg.KubeSecret != "" {
-		canPatch, err := checkSecretPermissions(ctx, cfg.KubeSecret)
+		canPatch, err := kc.CheckSecretPermissions(ctx, cfg.KubeSecret)
 		if err != nil {
 			log.Fatalf("Some Kubernetes permissions are missing, please check your RBAC configuration: %v", err)
 		}