refactor help func in CLI

Signed-off-by: Victor Vieux <victorvieux@gmail.com>

command/cli.go

@@ -20,6 +20,7 @@ import (
 	dopts "github.com/docker/docker/opts"
 	"github.com/docker/go-connections/sockets"
 	"github.com/docker/go-connections/tlsconfig"
+	"github.com/spf13/cobra"
 	"golang.org/x/net/context"
 )
 
@@ -73,6 +74,13 @@ func (cli *DockerCli) In() *InStream {
 	return cli.in
 }
 
+// ShowHelp shows the command help.
+func (cli *DockerCli) ShowHelp(cmd *cobra.Command, args []string) error {
+	cmd.SetOutput(cli.err)
+	cmd.HelpFunc()(cmd, args)
+	return nil
+}
+
 // ConfigFile returns the ConfigFile
 func (cli *DockerCli) ConfigFile() *configfile.ConfigFile {
 	return cli.configFile