Add CMS configuration for CERN

This is a response to https://cern.service-now.com/service-portal/view-request.do?n=RQF1495683, adding the CERN-specific configuration for cms.cern.ch to the container such that the CMS local site (CMS_LOCAL_SITE) is set to T2_CH_CERN. You can find more details on this at https://gitlab.cern.ch/clange/cms-cvmfs-csi/ (mind that the patch in that repository aims at kubernetes deployments, but the gist is the same). To confirm that this has the desired effect, the symbolic link /cvmfs/cms.cern.ch/SITECONF/local should point to /cvmfs/cms.cern.ch/SITECONF/T2_CH_CERN or simply T2_CH_CERN since local and T2_CH_CERN are in the same directory. If the site isn't set correctly, the symbolic link exists, but points to .. In the latter case, one cannot run CMSSW test jobs.