Support ELN composes
We want to try testing this in openQA. To implement that, we
need support in fedfind. So, here it is!
The compose has to be identified by its compose number, as we
can't figure out the URL from the compose ID or label (or any
of fedfind's silly representations of parts of those things).
Well, we probably *could*, using datagrepper, but that's
a bunch of unnecessary work, all we really *need* to be able
to do is get an instance from the URL (which we'll get out of
Signed-off-by: Adam Williamson <email@example.com>