add copyMVA parameter in MultiTrackMerger (forward port of #6535)

add copyMVA parameter in MultiTrackMerger
in order to handle also track collections not associated to any MVA value map 
(i.e., some track collection used @HLT as hltL3TkTracksMergeStep1 which is merging hltL3TkTracksFromL2OIState and hltL3TkTracksFromL2OIHit)

the default value is set to True,
=> offline configurations are fine (runTheMatrix -l selected run smoothly: 7 7 6 6 4 tests passed, 0 0 0 0 0 failed)

the HLT config has to --finally-- migrate to use TrackListMerger instead of SimpleTrackListMerger,
but the current HLT config is compatible (indeed, runTheMatrix ran ;) )

This is a forward port of #6535 and will be merged automatically since the former is fully signed for CMSSW_7_3_X.