IPA SUDO: download externalUser attribute
    
    This allows configuration with id_provider = proxy
    and sudo_provider = ipa when someone needs to fetch
    rules for local users.
    
    https://fedorahosted.org/sssd/ticket/2972
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 991c9f47fcb24704b880f60ab8ee77cfda056e2c)