Properly handle case where default number of threads requested

If a user requests the default number of threads by specifying '0' as the threads number, be sure to propagate the actual number of threads chosen by TBB to the rest of the system.
Merge pull request #6786 from Dr15Jones/recordNumThreadsIfDefaultUsed

Properly handle case where default number of threads requested
Merge remote branch 'origin/CMSSW_7_4_X' into CMSSW_7_0_X