Added QCD flat Pt and eta Pythia8 configuration files

Pythia8 configuration files for QCD samples flat in both Pt and eta. These samples will be used for BTV POG studies for the Phase 2 technical proposal. The config files require updated Pythia8 interface with additional User Hooks implemented. The necessary updates have been included in the following pull requests

https://github.com/cms-sw/cmssw/pull/2984 (pending)
https://github.com/cms-sw/cmssw/pull/3085 (merged)