archive opening should be a no-op, opening a stable release should be the op

[Steve Langasek]

For several years now we have had a 'devel' alias in the Ubuntu archive that has made it possible to treat devel as a rolling release, but in practice we are not consuming it in most places. This makes archive opening awkward because things have to be bootstrapped for e.g. autopkgtest cloud images to have the correct series listed etc.

In principle we could have 'devel' autopkgtest images in nova which always refer to the 'devel' alias in /etc/apt/sources.list and which continue to roll across the release date. Creating images for the next stable release is much easier since their creation does not have to block on the opening of stable+1 for uploads etc, and indeed the 'stable' autopkgtest image for a release can be pre-created before the release is marked stable in LP so that they're ready to go for SRUs on day 1.

A recent complicating factor are the changes to autopkgtest to try to accomodate Ubuntu vs. Debian differences in the "unstable" vs. "$release-proposed" approach to britney (partial suites).