Code review comment for lp:~joetalbott/core-image-builder/add_build_logic

I've logged several comments, but nothing that should block this current MP. In fact, I would feel better landing this and then iterating on the comments in other MPs.

Also, it is my opinion that the channel and release input needs to come from the core-tester-agent. This is to solve the problem of needing to test both vivid and wily packages that land into two different core channels. Perhaps there is another solution that I've overlooked.