openonuAdapterGo: key change for storing MIB templates in kv-store

There is a small legacy from the Python version of the adapter:
From it the triple vendor ID/equipment ID/SW version was introduced for addressing the location of MIB templates in the kv store.
Actually the version of the ONU from the ME ONU-G instead of the equipmentID from the ME ONU2-G would be a better choice, because the version is a mandatory parameter, while the equipmentID is only optional (see G.988). Therefore, the location for storing MIB templates in kv-store should be changed to the triple vendorID/ONU version/SW version.