Premixing workflows/customize redefined for 74x

[davidlange6]

On top of the recent PR from Mike, I have

• re-enabled the workflows for pre-mixing (though the ones that need pre-mixed input will continue to fail until that input is properly defined
• reorganized the customize functions so that they can be easily maintained. Mike is needed to check that I got all of the useful differences with respect to the standard ones - there were very few.

[cmsbuild]

A new Pull Request was created by @davidlange6 (David Lange) for CMSSW_7_4_X.

Premixing workflows/customize redefined for 74x

It involves the following packages:

Configuration/PyReleaseValidation
SLHCUpgradeSimulations/Configuration
SimGeneral/DataMixingModule
SimGeneral/MixingModule
SimTracker/SiStripDigitizer

@civanch, @nclopezo, @boudoul, @franzoni, @mdhildreth, @cmsbuild, @srimanob can you please review it and eventually sign? Thanks.
@ghellwig, @Martin-Grunewald, @wmtan, @makortel, @GiacomoSguazzoni, @jlagram, @rovere, @VinInn, @appeltel, @cnuttens, @prolay, @cerati, @venturia, @threus, @dgulhan, @LBeck this is something you requested to watch as well.
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[cmsbuild]

Pull request #7102 was updated. @civanch, @nclopezo, @boudoul, @franzoni, @mdhildreth, @cmsbuild, @srimanob can you please check and sign again.

[davidlange6]

please test