feat: ucind: implement cluster list command (#316)

* ucind: implement cluster list

I keep forgetting my ucind clusters, that add the missing list command:
```
% ./ucind cluster list
NAME    MACHINES
miek1   machine-1
```

Just the name would have been enough, but to actually make use of the
tui table a machines list is handy as well.
(There isn't much bubbletea in ucind, but I thought lets start now).

Signed-off-by: Miek Gieben <miek@miek.nl>

* Filter directly when listing

Filter on ucind managed while listing clusters

Signed-off-by: Miek Gieben <miek@miek.nl>

---------

Signed-off-by: Miek Gieben <miek@miek.nl>

cmd/ucind/cluster/root.go

@@ -13,7 +13,7 @@ func NewRootCommand() *cobra.Command {
 	}
 	cmd.AddCommand(
 		NewCreateCommand(),
-		// NewListCommand(),
+		NewListCommand(),
 		NewRemoveCommand(),
 	)
 	return cmd