Update an RDS parameter group.

    CLI example::

        salt myminion boto_rds.update_parameter_group my-param-group \
                parameters='{"back_log":1, "binlog_cache_size":4096}' \
                region=us-east-1

def update_parameter_group(name, parameters, apply_method="pending-reboot",
                           tags=None, region=None, key=None, keyid=None,
                           profile=None):
    '''
    Update an RDS parameter group.

    CLI example::

        salt myminion boto_rds.update_parameter_group my-param-group \
                parameters='{"back_log":1, "binlog_cache_size":4096}' \
                region=us-east-1
    '''

    res = __salt__['boto_rds.parameter_group_exists'](name, tags, region, key,
                                                      keyid, profile)
    if not res.get('exists'):
        return {'exists': bool(res), 'message':
                'RDS parameter group {0} does not exist.'.format(name)}

    param_list = []
    for key, value in six.iteritems(parameters):
        item = odict.OrderedDict()
        item.update({'ParameterName': key})
        item.update({'ApplyMethod': apply_method})
        if type(value) is bool:
            item.update({'ParameterValue': 'on' if value else 'off'})
        else:
            item.update({'ParameterValue': str(value)})  # future lint: disable=blacklisted-function
        param_list.append(item)

    if not param_list:
        return {'results': False}

    try:
        conn = _get_conn(region=region, key=key, keyid=keyid, profile=profile)
        if not conn:
            return {'results': bool(conn)}

        res = conn.modify_db_parameter_group(DBParameterGroupName=name,
                                             Parameters=param_list)
        return {'results': bool(res)}
    except ClientError as e:
        return {'error': __utils__['boto3.get_error'](e)}