Move standalone CMSSW installation to /cvmfs

Created by: clelange

In order to be fully exchangeable/transparent w.r.t. the containers that mount CVMFS, the full CMSSW should be moved to /cvmfs/cms.cern.ch. This probably also means changing the cmsset_default.sh script (to be investigated).

The best way of doing this is probably to just perform a multi-stage build in which the second step copies the contents of /opt/cms to /cvmfs/cms.cern.ch. Details to be found out.