Have the command line arg -n override numberOfThreads in config

If the command line argument ‘-n’ is used, it will be injected into the top level PSet, overriding any value within the configuration. This is done so EventProcessor can see the value and do the appropriate actions.
Merge pull request #4023 from Dr15Jones/haveCommandNOverrideConfig

Have the command line arg -n override numberOfThreads in config
Merge remote branch 'origin/CMSSW_7_2_X' into CMSSW_7_0_X