sysdb: add parent_dom to sysdb_get_direct_parents()
    
    Currently sysdb_get_direct_parents() only return direct parents from the
    same domain as the child object. In setups with sub-domains this might
    not be sufficient. A new option parent_dom is added which allows to
    specify a domain the direct parents should be lookup up in. If it is
    NULL the whole cache is searched.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 3dd4c3eca80e9223a65f3318821bd0fb5b45aedd)
    (cherry picked from commit 9a243dcdbf5a908d23c1a64f3fb33914eefef9e8)