Two minor fixes in Selections

ConstantMomentumScaling: likely due to new features in Gaudi(Python) the line from GaudiPython.Bindings import gbl as cpp produce error. Fixed now, replacing with more simple and more robust line
for MCTruthSelection downgrade the logging from info to debug

This MR is for run2-patches branch, but it will be nice to propagate it also to the master branch

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