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: 5aa5a1cb008963ee6a99011b8f7cd1f489e2bc6a
Component: engine
9c84f95c6e