'Associates a route table with a subnet within a VPC.
Note: A route table can be associated with multiple subnets.
:param      route_table: The route table to associate.
:type       route_table: :class:`.EC2RouteTable`
:param      subnet: The subnet to associate with.
:type       subnet: :class:`.EC2Subnet`
:return:    Route table association ID.
:rtype:     ``str``'

def ex_associate_route_table(self, route_table, subnet):

    params = {'Action': 'AssociateRouteTable', 'RouteTableId': route_table.id, 'SubnetId': subnet.id}
    result = self.connection.request(self.path, params=params).object
    association_id = findtext(element=result, xpath='associationId', namespace=NAMESPACE)
    return association_id
