files: mount EOSPUBLIC and serve file directly

[tiborsimko]

Following the recent PaaS upgrade, it is now possible to mount EOSPUBLIC volumes via FUSE onto PaaS applications. We have successfully tested this on the LHCb Ntupling Service.

Let us mount EOSPUBLIC volumes in the open data portal PaaS application and let us benchmark serving files directly from EOSPUBLIC without need to use (i) nginx reverse proxy or (ii) XRootD file serving.

This should hopefully lead to more efficient file serving with improved stability.