Fix heavy ion relval tests

[wmtan]

These are bug fixes for the failures of relvals 300.0, 301.0, and 302.0 in 7_6_X and 8_0_X.
These are bug fixes, so they are being submitted to 7_6_X.
There are three parts to the fixes:

1. Generalize the mixing module (mainly MixingWorker) to handle multiple labels at once for HepMCProduct, as the heavy ion smearer uses a smeared HepMCProduct from pileup to smear the unsmeared HepMCProduct from the signal. Do this so that the labels are not hard coded, but taken from the configuration files.
2. Fix heavy ion specific configurations to use the correct labels.
3. Fix one other config file ( SimMuon/GEMDigitizer/python/customizeGEMDigi.py) to properly handle backward compatibility.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_6_X.

Fix heavy ion relval tests

It involves the following packages:

GeneratorInterface/HiGenCommon
PhysicsTools/HepMCCandAlgos
SimGeneral/MixingModule
SimMuon/GEMDigitizer

@vciulli, @civanch, @covarell, @mdhildreth, @monttj, @cmsbuild, @thuer, @bendavid, @vadler, @govoni can you please review it and eventually sign? Thanks.
@cbaus, @mkirsano, @jhgoh this is something you requested to watch as well.
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[wmtan]

@yetkinyilmaz FYI, this fixes the HI relval tests.

[civanch]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9509/console

[cmsbuild]

+1
Tested at: 0e6b031
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12303/9509/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12303/9509/summary.html

[wmtan]

While looking at this PR, I found a minor bug that would cause an incorrect LogInfo message to be issued. So, I updated this PR with a second commit to fix this bug.

[cmsbuild]

Pull request #12303 was updated. @vciulli, @civanch, @covarell, @mdhildreth, @monttj, @cmsbuild, @thuer, @bendavid, @vadler, @govoni can you please check and sign again.

[wmtan]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9513/console

[cmsbuild]

+1
Tested at: e558345
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12303/9513/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12303/9513/summary.html

[covarell]

+1

[civanch]

+1