updated to deal with the migration of CouchDB server (LBCORE-944)
- modified cron/clean_up_reduced_db.sh to work on lbcouchdb.cern.ch too
- modified the URLs to point to lhcb-nightlies.cern.ch or lbcouchdb.cern.ch, depending on the use case)
- updated documentation
Fixes LBCORE-944.