Make default context behaves like a real context:

- when using "--context default" parameter
- when printing the list of contexts
- when exporting the default context to a tarball

Signed-off-by: Jean-Christophe Sirot <jean-christophe.sirot@docker.com>
(+1 squashed commit)
Squashed commits:
[20670495] Fix CLI initialization for the `docker stack deploy --help` command and ensure that the dockerCli.CurrentContext() always returns a non empty context name (default as a fallback)
Remove now obsolete code handling empty string context name
Minor code cleanup

Signed-off-by: Jean-Christophe Sirot <jean-christophe.sirot@docker.com>

cli/command/stack/cmd.go

@@ -48,6 +48,10 @@ func NewStackCommand(dockerCli command.Cli) *cobra.Command {
 	}
 	defaultHelpFunc := cmd.HelpFunc()
 	cmd.SetHelpFunc(func(c *cobra.Command, args []string) {
+		if err := cmd.Root().PersistentPreRunE(c, args); err != nil {
+			fmt.Fprintln(dockerCli.Err(), err)
+			return
+		}
 		if err := cmd.PersistentPreRunE(c, args); err != nil {
 			fmt.Fprintln(dockerCli.Err(), err)
 			return