Allow filtering by arch when scheduling compose jobs
Sometimes it's useful to run/re-run compose jobs only for one
or more specific arches, so let's allow this both via API and
CLI. `jobs_from_compose` has a new kwarg which is a list of
arches to schedule jobs for, and the CLI has a `--arches` arg
that feeds it (with a comma-separated list).
Signed-off-by: Adam Williamson <firstname.lastname@example.org>