Add an API route that returns logs for a specific compose.
For now, this contains the result of the job, in JSON. The idea is to
put more and more of this information into structured APIs. This is a
first step to make logs available at all.
Amend koji-compose.py to check that the route exist and contains as many
"image_logs" as images that were requested (currently always 1).
Based on a patch by Chloe Kaubisch <chloe.kaubisch@gmail.com>.
e47b44329e