Introduce separate gNB-DU ID in configuration

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.

ci-scripts/conf_files/gnb-du.sa.band78.106prb.usrpb200.conf

@@ -7,6 +7,7 @@ gNBs =
  {
     ////////// Identification parameters:
     gNB_ID = 0xe00;
+    gNB_DU_ID = 0xe00;
 
 #     cell_type =  "CELL_MACRO_GNB";