The value of the object tmnxL2tpTgCfgSecret specifies
the password to be used for the authentication of the tunnels in this
Layer Two Tunneling Protocol Tunnel Group.
For any given tunnel in this group, the value of tmnxL2tpTgCfgSecret
can be overruled with the value of tmnxL2tpTuCfgSecret.
This object will always return an empty string on a read.