Extend LHEStepZero support

Include a new option that indicates if we are processing
lhe input files instead of forking a generator in cmsRun,
this way the input source options are set correctly
to read n events per job.

Fixes #2090
Merge pull request #4567 from dballesteros7/LHEStepZero-Reloaded

Extend LHEStepZero support