sdap: properly handle binary objectGuid attribute
    
    Although in the initial processing SSSD treats the binary value right at
    some point it mainly assumes that it is a string. Depending on the value
    this might end up with the correct binary value stored in the cache but
    in most cases there will be only a broken entry in the cache.
    
    This patch converts the binary value into a string representation which
    is described in [MS-DTYP] and stores the result in the cache.
    
    Resolves https://fedorahosted.org/sssd/ticket/2588
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>