bin/copy-image should support setting an initial phase percentage for phased updates

[Steve Langasek]

When you run copy-image, the default phased percentage is 100%. If you want less than 100% phasing, you have to run a second set-phased-percentage command to override. This means running two commands where you should be able to run one, and also means there's a risk of a race condition with the publisher causing the image to be pushed to all users before set-phased-percentage has been run.

The copy-image command should take an optional "initial phase percentage" option.

[Steve Langasek (vorlon)]

Relatedly, I think etc/config should support an initial phasing setting. This would be used by the importer to, e.g., let us set 0% phasing for images imported onto the devel-proposed channel, with the phasing percentage only being increased once the image has passed QA smoke testing.

[Łukasz Zemczak (sil2100)]

The initial phasing setting should be, of course, per-channel. That makes sense indeed, although we would probably only really start using it only after we have automatic smoketesting set-up from the QA side. Or, at least some manual process similar to smoketesting.