add support to stageout from HLT Cloud at CERN

The VMs on the HLT Cloud at CERN don't have access to the EOS environment. Change the stageout plugin to only use generic xrootd commands.

Patch was developed by Stephen Gowdy, I did test it in a replay though. The replay only ran against EOS though, so we still need a larger replay that also tests castor stageout.

This isn't urgent and can happen after the HI run is over. Any kind of production wmagent
release for HLT Cloud operation would need to include this patch though.