extend cmsDriver with --premixed_input for pre-mixed files in input to DATAMIX
re-use pileup_input, in place of premixed_input. Remove unnecessary check
put back original comment
remove option
Merge pull request #3000 from franzoni/premixed_input-cmsDriver-from-CMSSW_7_0_1

cmsDriver handles pre-mixed input to datamixer
Merge remote branch 'origin/CMSSW_7_0_X' into CMSSW_7_0_X