The value of the object tmnxL2tpAccountingPolicy specifies
the RADIUS accounting policy to use in this L2TP protocol instance.
It refers to a conceptual row in the tmnxL2tpApTable.
The value of tmnxL2tpAccountingPolicy can be overruled by
a non-empty value of tmnxL2tpTgCfgAccountingPolicy
or tmnxL2tpTuCfgAccountingPolicy.