buildsys: Set _koji_hub when setting up the buildsys
buildsys: Authenticate sessions without caching them

Commit 9727694c63481d5d520204532b184e52b8bfa98c disabled reusing logged
in Koji sessions because they were expiring instantly.

However, it also disabled logging in to Koji.

This commit reintroducing Koji authentication, without any caching
mechanism at all.
Merge pull request #65 from bochecha/kojiauth

Fix authentication of Koji sessions