fleetctl: inform the user about the '-replace' switch in case the units differ

[tixxdz]

If the unit in the registry is different from the one that is on disk,
and the user did not provide the '--replace' switch flag, then instead
of just printing a warning, let him know about the 'replace' flag.

Closes: #614

[tixxdz]

This PR depends on #1509

[dongsupark]

Looks good. A simple test also works.

[antrik]

I think it would be better to just merge this into the other PR.

Also, this surely should be covered by some functional test? As you didn't have to change the expected result of any existing tests, I assume there is none yet for this case, i.e. a new one needs to be added...

[tixxdz]

@antrik, sorry but that's not a good idea, each PR on its own now. Other PRs have already been delayed for several days for such cases or many comments without concluding lgtm or comments who lacked the right context of what the PR does... So I'm not adding more patches on top to complicate things.

[antrik]

@tixxdz let's not overgeneralise. In this case it really doesn't make sense to have it separate, as it just changes the code introduced by that other PR; and the change itself should be non-controversial -- so having it separate is just unnecessary code churn and reviewer burden. Just implement the desirable behaviour right away.

(Note that the other PR has not been reviewed by Jon yet -- so it's not like you would be loosing anything by adding improvements to it...)

[dongsupark]

@antrik
When I first saw this PR, I actually thought about adding functional tests for checking the warning message. But I didn't do it, because that looked like just one of minor things or nice-to-have. Moreover, think about what would happen if we change the warning message in fleetctl. We would then have to update the functional test again.

Apart from all of that, if you'd really like to see a new functional test for the warning message, sure, I could write it, as I'm also responsible for the functional test for the replace option.

I don't care, whether this PR should be a separate one or not. That's just a matter of taste. I'd just like to see the entire replace option implementation finally merged.