3GPP has the concept of a gNB-CU-UP ID and a (separate) gNB ID (for gNB,
CU-CP). This commit introduces the gNB-CU-UP as a separate ID that has to be
set in the configuration file when running as a CU-UP. For the
CU/monolithic gNB, it is optional (but needs to be the same as the
gNB-ID if specified).

The CU-UP ID is necessary, as some entities, e.g., the e2 agent, need to
signal both IDs, e.g., to reconcile a CU-CP and (multiple) CU-UP(s)
belonging together.