Update HTTP path for groupdata download

I happened to want to test out some files that are already in the GroupData http area but not replicated to cvmfs, from a location without access to EOS. Activating the HTTP download env variable surprised me by referring to the old url (http://atlas.web.cern.ch/Atlas/GROUPS/DATABASE/GroupData), rather than the new one (https://atlas-groupdata.web.cern.ch/atlas-groupdata).

It's possible that I have missed some discussions around this, so tagging a few ASG people: @vimartin @lheinric @krumnack. If the HTTP download is not intended to be supported I expect the option should be removed entirely, so it seems appropriate to keep the path current.