GlobalTag.py: sort the additional payloads

Sort the additional payloads from release-level or user-level customisations to the global tags,
to ensure a consistent hash of the process configuration.
Add flat PU 20-50, 50ns
Merge pull request #4808 from srimanob/TSG707

Add flat PU 20-50, 50ns
Merge pull request #4731 from fwyzard/GlobalTag_fixed_order_70X

GlobalTag.py: sort the additional payloads
changing input collection for tauGenJets in miniAOD
fix electron source PFCand pointers
equivalent fix also for photons
Merge pull request #4833 from jpavel/70x_taus_miniAOD_tauGenJetInput

Changing input collection for tauGenJets in miniAOD
Merge pull request #4862 from gpetruc/fixEleSourcePFCands-fromCMSSW_7_0_6_patch1

fix pat::Electron source PFCand accessor in miniAOD