Clarify the use of `YumRepository`

[ydirson]

We're getting into a case where performInstallation.add_repos() gets passed a YumRepository instance in repos.

I guess this happens because Accessor.findRepository will instanciate a YumRepository (since 32d90da) if a transient network issue will cause isRepo() for both UpdateYumRepository and MainYumRepository to fail, and then ac0d438 assumes everything that is not a UpdateYumRepository is a MainYumRepository.

What do you think @alexbrett ?