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
implement a copy-delete type subscription

Normally there are only two subscription modes supported, copy or move.
This implements a delayed move, consisting of a copy first, followed by
a delete from the source site once the workflow creating the data is done.

Will check for existing dataset or block subscriptions before doing the delete.