Prevent client from reload()-ing CMSSW configs

CMSSW configs don't like to be imported multiple times in the same
process. The better solution would be to fork() out the part that
handles the configuration handling, but in the short-term, this commit
takes care of the "easy" case
Merge pull request #4249 from PerilousApricot/bugfix/dont_import_multiple_times

Prevent client from reload()-ing CMSSW configs