Compose scheduling: allow arch filtering in config file
    
    This lets you have a WANTED config (or use the default) that
    specifies arches your instance doesn't have workers for, and
    filter them out via the config file. Of course you could instead
    have a custom WANTED that doesn't include the arches you don't
    want, but that's kinda a lot more work to maintain. The goal
    here is for infra prod and lab instances to both just use the
    stock WANTED, but filter out arches they don't have workers for,
    instead of needing to use a custom WANTED config file.
    
    We also expand the stock WANTED to include all arches, but make
    the default setting of the config option "x86_64" to match
    previous behaviour (when stock WANTED had only x86_64 images) so
    folks with their own instances don't suddenly get a bunch of
    aarch64 and ppc64le jobs they can never fill.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>