Merge ~cjwatson/launchpad:livefs-build-pocket into launchpad:master

Allow livefs build metadata to override the default build pocket

This makes it possible, for example, to distinguish the case of building a livefs using tools from -updates but with contents from the release pocket from the case of building a livefs using tools from -updates and contents from -updates.

The use case here is that at the moment buildd images are built with tools from -updates but with contents from the release pocket. I'd like it to be possible to build buildd images with contents from -updates as well, since we could use those for builds against -updates and it would save a good deal of time at the start of each build.

We've ended up in a bit of a compatibility corner due to some unfortunate past decisions (mostly by me), and this is the least bad solution I can think of. launchpad-buildd and livecd-rootfs will also need some work to support this, but this design allows it to be done in a somewhat backwards-compatible way (that is, it wouldn't break existing livecd-rootfs code). The one compatibility issue is that existing livefses that intend to continue building with tools from -updates and contents from the release pocket (or similar) will need to have "pocket": "release" explicitly added to their metadata before landing the corresponding launchpad-buildd change which starts paying attention to the "pocket" argument; we'll need to go through the current database to check for that kind of thing.

This is essentially the same as https://code.launchpad.net/~cjwatson/launchpad/livefs-build-pocket/+merge/372444, converted to git and rebased on master.