reduce gfal2 log verbosity

[rcarpa]

Motivation

We have messages like

Using bearer token for HTTPS request authorization

and

[gfal_module_load] plugin /usr/lib64/gfal2-plugins//libgfal_plugin_xrootd.so loaded with success

spamming our logs millions of times per day in reaper

I didn't find any useful information in gfal logs on INFO level. Whatever is really useful is already extracted by rucio and printed in rucio log messages. So probably good idea to move to warning in gfal2 logs.