[auth] pass `repo` instead of `vreg` to SessionManager and AuthenticationManager
Those object actually need a repo object to connect and create new session
object. They used to retrieve the repo object using
``vreg.config.repository()``. I'm trying to sanitise the initialisation stack
and get ride of ``config.repository()``, a function that mix factory and cache.
Retrieving the vreg from the repo is trivial (``repo.vreg``) so we now pass the
repo object instead of the vreg.