Ubuntu Core images should have separately counted image datestamps per channel

[Łukasz Zemczak]

Currently it looks like all ubuntu-core images for a given series (16, 18) share a common image publishing timestamp count, regardless of channel. This is due to the architecture of cdimage, where only every series/project/image-type gets their own numbering. And since ubuntu-core CHANNEL=edge and CHANNEL=beta are basically using the same series/project/image-type, common numbering is used.

But this results in a bit of an ugly thing after we enabled beta-based ubuntu-core daily images, where we always end up with a .1 in the datestamp version because of running 'twice' for the same project (once for edge and once for beta).

It would be nice if CHANNEL was handled like a separate thing. It makes logical sense as currently different CHANNEL images anyway end up in a different, separate per-channel directory, so it's already 'special-cased' in cdimage. We should extend that to now also not count them as one and have each channel have its own image date-based numbering.

One thing we'd need to remember is to also make sure log files don't conflict. Currently they're in ubuntu-core/<series>/daily-live-<datestamp>.log, so maybe we'd want to go with channel in either a subdirectory or maybe putting it in the logfile name.