Support skipping missing channels in promote train workflow

[MichaelThamm]

We should update this workflow logic so that we can still promote train when a channel is missing in a charm.

Otelcol was missing the beta channel and was therefore not getting its stable channel promoted. This will happen in more places if we forget to release to a channel.

Alternatively, we should add a regular check that our repos have all channels so we do not miss promotion trains. Intentionally adding a missing channel can be a nice way to stop the automatic promotion.

[lucabello]

This check is more complex than it appears to be, because there's no clear way to tell whether a missing channel is intentional or not. I would like to avoid a workflow where for each charm we introduce (more) complex logic around checking "the next channel is closed, is the one two risks ahead open? if not, what about three risks ahead?" .

Currently what we're practicing is release to all channels (not sure why some of your repos are doing it differently), and the CI is tailored to that. I think we should uniform the way we release our projects, so I would encourage everyone to stick with that until we discuss a change for all of us :)

This problem will go eventually away with quality gates handling the promotion process; according to our most-recent plans, we'd still make use of all risk channels - so we should stick to using all of them now as well.

IMHO I would close this issue, but I don't mind leaving it open until we find a different solution.