Pixel DQM booking MonitorElements string update

Updating the folders of the MonitorElements in Pixel DQM packages. In the future a second layer of DQM process will go on at HLT level, and this would conflict with the original one's folders. In order to avoid that, the base folders of the MonitorElements are set in the python configuration files. The necessary modifications for this can be seen in the PR.

This is a forward port of #6569 into CMSSW_7_4_X. Merging since the former is fully signed in CMSSW_7_3_X.