gracefully handle setting replica bind dn group on old masters
    
    Pre-3.3 masters do not support setting 'nsds5replicabinddngroup'
    attribute on existing replica entry during setup of initial replication.
    In this case UNWILLING_TO_PERFORM is returned. The code can interpret
    this error as an indication of old master and fall back to just adding
    its LDAP principal to entry's 'nsds5replicabinddn' attribute.
    
    https://fedorahosted.org/freeipa/ticket/6532
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>