Create a new Subnet

        :type vpc_id: str
        :param vpc_id: The ID of the VPC where you want to create the subnet.

        :type cidr_block: str
        :param cidr_block: The CIDR block you want the subnet to cover.

        :type availability_zone: str
        :param availability_zone: The AZ you want the subnet in

        :rtype: The newly created Subnet
        :return: A :class:`boto.vpc.customergateway.Subnet` object

def create_subnet(self, vpc_id, cidr_block, availability_zone=None):
        """
        Create a new Subnet

        :type vpc_id: str
        :param vpc_id: The ID of the VPC where you want to create the subnet.

        :type cidr_block: str
        :param cidr_block: The CIDR block you want the subnet to cover.

        :type availability_zone: str
        :param availability_zone: The AZ you want the subnet in

        :rtype: The newly created Subnet
        :return: A :class:`boto.vpc.customergateway.Subnet` object
        """
        params = {'VpcId' : vpc_id,
                  'CidrBlock' : cidr_block}
        if availability_zone:
            params['AvailabilityZone'] = availability_zone
        return self.get_object('CreateSubnet', params, Subnet)