'Figures out what the default ACL is for the VPC, and associates
current network ACL with the default.
:type subnet_id: str
:param subnet_id: The ID of the subnet to which the ACL belongs.
:type vpc_id: str
:param vpc_id: The ID of the VPC to which the ACL/subnet belongs. Queries EC2 if omitted.
:rtype: str
:return: The ID of the association created'

def disassociate_network_acl(self, subnet_id, vpc_id=None):

    if (not vpc_id):
        vpc_id = self.get_all_subnets([subnet_id])[0].vpc_id
    acls = self.get_all_network_acls(filters=[('vpc-id', vpc_id), ('default', 'true')])
    default_acl_id = acls[0].id
    return self.associate_network_acl(default_acl_id, subnet_id)
