pkgset: Add option to ignore noarch in ExclusiveArch
    
    The `add_noarch` option of `get_valid_arches` is broken and doesn't
    really do anything (noarch is always present in the result).
    
    This causes packages that have ExclusiveArch including noarch to
    actually not be excluded. They should be.
    
    Changing this globally could have a very big impact. Therefore we can
    hide it behind a configuration option so that it's opt-in.
    
    JIRA: COMPOSE-2457
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>