Base._conf lasts the lifetime of Base and can be passed via constructor.

Treating base.conf as such can speed up the tests. Also opens
opportunities for simplifying code at places as objects can reference the
config without having to keep a Base reference.

Related:RhBug:1117789
api: better describe how Repos should be created, example. (RhBug:1117789)
api: In Repo(), cachedir is a required argument.

It has been this way since introducing the call, but incorrectly documented.