Add support for default image from /usr/lib/os-release or so

With `system-reinstall-bootc` one thing that might be nice for operating systems to support is basically having the user not provide a container image could default to a generic one (or one specific to the image type).

We've talked in the past for example about having a pre-built "cloud" bootc image and I can imagine that firing up e.g. c9s in AWS and just typing `system-reinstall-bootc` would default to that image. In this flow we also wouldn't do any of the SSH key injection because we can assume the target image has cloud-init. The goal here is to make this as close to "flip this system over to bootc" as possible.

I'm thinking we could have something like
`BOOTC_IMAGE=quay.io/centos-bootc/centos-bootc-cloud:stream9` in `/usr/lib/os-release` or so.

(Tangential but if we improve https://containers.github.io/bootc/booting-local-builds.html then I think we could even encourage a flow where the user tests things out that way, i.e. they just boot a cloud image and do s-r-b, then test some local builds, then move to git-ops etc.)