Add option to not clean the env in Scram.py

The scriptExe functionality of CRAB3 is using this library
(Scram.py), but it would be nice to avoid cleaning the environment so that
variables like X509_USER_PROXY can be used for example.
Notice that the CMSSW.py executors also does not clean the env for
cmsRun.
Merge pull request #5525 from mmascher/master

Add option to not clean the env in Scram.py