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.
Merge pull request #4401 from hufnagel/cernhltstageout

add support to stageout from HLT Cloud at CERN