Show usage when docker swarm update has no flags

This fix tries to address the issue raised in 24352. Previously,
when `docker swarm update` has no flags, the output is
```
Swarm updated.
```
even though nothing was updated. This could be misleading for
users.

This fix tries to address the issue by adding a `PreRunE` function
in the command so that in case no flag is provided (`cmd.Flags().NFlag() == 0`),
the usage will be outputed instead.

An integration has been added to cover the changes.

This fix fixes 24352.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 249d4e5709
Component: cli

components/cli/command/swarm/update.go

@@ -23,6 +23,12 @@ func newUpdateCommand(dockerCli *command.DockerCli) *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runUpdate(dockerCli, cmd.Flags(), opts)
 		},
+		PreRunE: func(cmd *cobra.Command, args []string) error {
+			if cmd.Flags().NFlag() == 0 {
+				return pflag.ErrHelp
+			}
+			return nil
+		},
 	}
 
 	cmd.Flags().BoolVar(&opts.autolock, flagAutolock, false, "Change manager autolocking setting (true|false)")