Adding first LHE sequence to RelVal 

This pull request include the first RelVal workflows with LHE for DY->ll+Jets. Moreover, following the request of Josh, the jet matching has been changed to use the new pythia8 jet matching for the LHE workflows. All of RelVal sequences that use LHE files in the generator and extended generator workflows have had the a fragment added for easy use in the future. The full generator and extended RelVal workflows have been tested on a small number of events. 
Automatically ported from CMSSW_7_3_X #6554
@franzoni does this mean 250k events total with 20k per lumi (and therefore 20k per job).  What is the target job length for relvals?  Should we reduce this a bit?