O2DPG: Fix vertex inconsistency between background and signal MC events

Fixes https://its.cern.ch/jira/browse/O2-6368.

The background events still sampled the vertex position from CCDB
while it should have taken it from the pregenerated collision context.
This commit rectifies this.

Author: sawenzel

MC/bin/o2dpg_sim_workflow.py

@@ -714,7 +714,7 @@ def getDPL_global_options(bigshm=False, ccdbbackend=True):
                      + ' -g  '      + str(GENBKG) + ' '    + str(MODULES)  + ' -o bkg ' + str(INIBKG)                    \
                      + ' --field ccdb ' + str(CONFKEYBKG)                                                                \
                      + ('',' --timestamp ' + str(args.timestamp))[args.timestamp!=-1] + ' --run ' + str(args.run)        \
-                     + ' --vertexMode kCCDB'                                                                             \
+                     + ' --vertexMode ' + vtxmode_sgngen                                                                 \
                      + ' --fromCollContext collisioncontext.root:bkg '                                                   \
                      + ' --detectorList ' + args.detectorList