3GPP has the concept of a gNB-DU ID and a (separate) gNB ID (for gNB,
CU). This commit introduces the gNB-DU as a separate ID that has to be
set in the configuration file. It is necessary, as some entities, e.g.,
the e2 agent, need to signal both IDs, e.g., to reconcile a CU and
(multiple) DU(s) belonging together.