install: Support reinstallation (factory reset)

[cgwalters]

#137 is tracking part of this, a lot of related discussion in coreos/fedora-coreos-tracker#399

Basically we should support something like bootc install factory-reset that would configurably wipe local system state.

On the implementation side...I think the best way to do this would be something like:

• Create new deployment with fresh etc targeting a new stateroot
• Boot into it
• optional: clean up old deployment + stateroot

The beauty of this approach is that the "factory reset" would actually also be fully transactional, i.e. you could roll back into the previous state. It'd be easy to copy over any desired state (SSH host keys, cached container images e.g.) from the previous boot before deleting it.

[cgwalters]

BTW one tricky thing in this is kernel arguments...cc #401

And also related to this, we should probably go to some effort to preserve the original kargs.

---

An entirely different way to implement this would be to retain a pinned version of the original installed deployment. Some people will want to do that, and we already effectively support it with ostree admin pin. However...rolling back to that in the general case will imply rolling back to something pretty outdated, which we don't want to encourage. So I think we should focus more on better capturing and maintaining the bits of state that we want from that which mostly I think boils down to kargs.