Code review comment for lp://qastaging/~bogdana/landscape-client/save-aptsources

@Adam, ISTR we already use the client's filesystem to "store" disabled repos in /etc/apt/sources.list.d/*.list before setting up repository profiles using exactly the same approach (rename them to .save and that's it). I think making top-level sources.list behave the same is an improvement of the current state of things.