Giving an implementation to the DQM customizer function for EGM "old reco mode"

This PR is built on top of #1834, commit 7c96d0d. Relative change wrt the original PR is only in one file
RecoEgamma/Configuration/python/customizeOldEGReco.py
to give an implementation to the function customize_DQM. The function will modify the cluster collection names that ECAL DQM points to to the old EG clusters.