SUDO: Only store lowercased attribute value once
    
    The current code doesn't handle the situation where lowercasing the
    sudoUser attribute would yield the same value again.
    
    For example:
        sudoUser:   TUSER
        sudoUser    tuser
    would break.
    
    This patch switches to using the utility function
    sysdb_attrs_add_lower_case_string() which already checks for duplicates.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3301
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit a5ecc93abb01cece628fdef04ebad43bba267419)