Broccoli distributed auth

Broccoli needs to pass the correct credentials whenever it needs to fetch data
from textrepo (and annorepo). In general AR returns TR urls, so we might need
multiple TR auth details in the configuration?

Do we want a single, fixed TR per project? Or a map of "registered" TRs, where
broccoli then matches the TR urls from AR aginst this map to find out the
details?

What about per-user credentials?
