Adjustments for using MixingModule for Heavy Ion simulations

This is the minimum updates in order to be able to run MixingModule for Heavy-Ion embedding studies.
Presented in https://indico.cern.ch/event/344150/

The main difference from pp pile-up simulations is that it runs mixing in gen-level, and the sim-level mixing in next process happens in playback mode. Event contents are updated accordingly. ConfigBuilder is adjusted so that the commands here define the workflows successfully:
https://github.com/CmsHI/cmssw/blob/mix_74X_08/HeavyIonsAnalysis/Mixing/test/runChainTemp.sh

Tested on 
slc6_amd64_gcc491 
by
runTheMatrix.py -s --useInput all
runTheMatrix.py -l 140.53

To be followed up in later pull-requests:
- Clean up ConfigBuilder.py, StandardSequences other configs for left-overs form the old heavy-ion embedding workflow
- Define the RelVal workflows
- Update PyquenInterface also for the ability to read background event information (special embedding features of Pyquen are temporarily disabled)
- Update in PlaybackInfo dataformat to keep file info - independent development