'Modifies a cluster subnet group to include the specified list
of VPC subnets. The operation replaces the existing list of
subnets with the new list of subnets.
:type cluster_subnet_group_name: string
:param cluster_subnet_group_name: The name of the subnet group to be
modified.
:type description: string
:param description: A text description of the subnet group to be
modified.
:type subnet_ids: list
:param subnet_ids: An array of VPC subnet IDs. A maximum of 20 subnets
can be modified in a single request.'

def modify_cluster_subnet_group(self, cluster_subnet_group_name, subnet_ids, description=None):

    params = {'ClusterSubnetGroupName': cluster_subnet_group_name}
    self.build_list_params(params, subnet_ids, 'SubnetIds.member')
    if (description is not None):
        params['Description'] = description
    return self._make_request(action='ModifyClusterSubnetGroup', verb='POST', path='/', params=params)
