INSTALL-ubuntu docs update.

This reflects the discussion at the end of #1754 regarding the overriding of defaults that likely won't actually be changed. I created a separate file to copy that includes settings that most users will have to change; as well as the oversight on my part here: http://meta.discourse.org/t/maybe-some-mistakes-in-official-discourse-ubuntu-install-guide/11537/2

It'd be great if someone could give these a quick read before pushing, but I think I stomped everything out beforehand.