Giving implementation to customize_DQM

This PR is built on top of #1834 at commit b9f0ceb and changes only one file (RecoEgamma/Configuration/python/customizeOldEGReco.py).
It gives an implementation to the EGM customizer function to revert the cluster collection names in ECAL DQM to the old ones.

The previous PR of the same content has been removed and is superseded by this PR, due to a change in the base PR mentioned above.