Support 'all' in orchestrator flag for docker stack ls

All other docker stack commands report an error when passed this value.

Signed-off-by: Mathieu Champlon <mathieu.champlon@docker.com>

cli/command/stack/cmd.go

@@ -1,11 +1,15 @@
 package stack
 
 import (
+	"fmt"
+
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/spf13/cobra"
 )
 
+var errUnsupportedAllOrchestrator = fmt.Errorf(`no orchestrator specified: use either "kubernetes" or "swarm"`)
+
 // NewStackCommand returns a cobra command for `stack` subcommands
 func NewStackCommand(dockerCli command.Cli) *cobra.Command {
 	cmd := &cobra.Command{