CACHE_REQ: Descend into subdomains on lookups
    
    Let's make all plugins, but the "host_by_name", to descend into the
    subdomains on lookups.
    
    This patch basically prepares the field for the coming up patches that
    will allow group/user resolution in all domains (or a subset of the
    domains) to be possible by only using the short names without the domain
    component.
    
    The "host_by_name" plugin was not changed as it's a specific IPA plugin
    and won't find anything on its subdomains.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3001
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>