The value of the object tmnxL2tpTgCfgAccountingPolicy specifies
the RADIUS accounting policy to be used in this Layer Two Tunneling
Protocol Tunnel Group.
It refers to a conceptual row in the tmnxL2tpApTable.
The value of tmnxL2tpTgCfgAccountingPolicy can be overruled by
a non-empty value of tmnxL2tpTuCfgAccountingPolicy.
An empty string specifies that the value of tmnxL2tpAccountingPolicy
must be used.