metropolis/cli/metroctl: move takeownership under cluster subcommand tree

This is in preparation for having more cluster-wide commands (same as we
keep 'node' for all node-specific commands).

Change-Id: I3fa5935e1d751261bf82a80cf61e811fa36effc9
Reviewed-on: https://review.monogon.dev/c/monogon/+/3593
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/cli/metroctl/cmd_cluster.go b/metropolis/cli/metroctl/cmd_cluster.go
new file mode 100644
index 0000000..e82cb95
--- /dev/null
+++ b/metropolis/cli/metroctl/cmd_cluster.go
@@ -0,0 +1,12 @@
+package main
+
+import "github.com/spf13/cobra"
+
+var clusterCmd = &cobra.Command{
+	Short: "Manages a running Metropolis cluster.",
+	Use:   "cluster",
+}
+
+func init() {
+	rootCmd.AddCommand(clusterCmd)
+}