Beamspot nominal collision2015: set new default

- introduce beamspot parameters as agreed during PPD General discussion https://indico.cern.ch/event/341218/ , NominalCollision2015
- *the defaualt beamspot* (used by ConfigBuilder if no option is specified) is updated to be NominalCollision2015
- use the NominalCollision2015 for postLs1 relvals, keep Realistic8TeVCollision in runI relvals