Replace cout with LogWarning in MixCollection

ED products are not supposed to use std::cout. The static analyzer
found this one and lead to thread safety complaints for modules.
Switching to LogWarning solves the problem.
Merge pull request #6083 from Dr15Jones/removeCoutFromMixCollection

Replace cout with LogWarning in MixCollection
Merge remote branch 'origin/CMSSW_7_3_X' into CMSSW_7_0_X