CRAB 3 setup

Setup to run crab3. Please note that while this is supposedly correct, the full-fledge production test is still pending due to other ongoing issues, and that these may be hiding some fixes needed in the code.
- https://hypernews.cern.ch/HyperNews/CMS/get/computing-tools/141/1/2.html
- https://hypernews.cern.ch/HyperNews/CMS/get/computing-tools/147.html