introduce NominalCollision2015 beamspot scenario, according to: https://indico.cern.ch/event/341218/
beamspot of postLs1 relvals set to NominalCollision2015; run1 relvals left as they were, making it explicit Realistic8TeVCollision (note: Realistic8TeVCollision is still the cmsDriver default)
replace deafault beamspot to be NominalCollision2015 (was: Realistic8TeVCollision)
Merge pull request #5464 from franzoni/beamspot_NominalCollision2015-CMSSW_7_3_X_2014-09-22-0200

Beamspot nominal collision2015: set new default