sysdb: remove ghosts in all sub-domains as well
    
    If a user is a member is a group in a different sub-domain, e.g with
    universal groups in AD, the ghost attribute might not be properly
    removed from the group object if the user is resolved. The reason is
    that only groups from the domain of the user were search for ghost
    attributes. This patch increases the search-base to all sub-domains of
    the configured SSSD domain.
    
    Resolves https://fedorahosted.org/sssd/ticket/2567
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>