Snapcraft promote

[sergiusens]

@evan

I think an error with an override flag is reasonable. The original motivation I heard for promote was to address the case where automation gives you architecture builds in edge and you forget to the promote all but the one you use (amd64, typically). If the version strings are different, that is an indication but not confirmation that these revisions in edge are unrelated.

I believe that it should promote exactly what is on those channels and that the visual confirmation is the queue to consider moving forward or not.

I do not wish to overload promote when release <snap-name> <revision> <channel> is perfectly capable of this. We could try and be smarter in the future when we can tag these builds and provide hints into maybe wanting to use the release command instead right before the yes/no prompt.