Helper function verify a subnet's route table association

    route_table_desc
        the description of a route table, as returned from boto_vpc.describe_route_table

    subnet_id
        the subnet id to verify

    .. versionadded:: 2016.11.0

def _verify_subnet_association(route_table_desc, subnet_id):
    '''
    Helper function verify a subnet's route table association

    route_table_desc
        the description of a route table, as returned from boto_vpc.describe_route_table

    subnet_id
        the subnet id to verify

    .. versionadded:: 2016.11.0
    '''
    if route_table_desc:
        if 'associations' in route_table_desc:
            for association in route_table_desc['associations']:
                if association['subnet_id'] == subnet_id:
                    return True
    return False