FNAL staging: do not invoke mkdir for xrootd

xrdcp auto-creates parent directories, so we don't need any
logic in CreateOutputDirectory for xrootd.

The previous version worked only if the filesystem was also mounted
locally -- that was OK for production which has /lustre/unmerged mounted,
but not for CRAB3 and /store/temp)
Merge pull request #5207 from holzman/master

FNAL staging: do not invoke mkdir for xrootd