Support coreos.volatile karg

[cgwalters]

Moving this over from coreos/ignition#935

Basically let's support something like systemd.volatile (docs) that actually works on CoreOS (because we'd copy /etc rather than have it start empty, among other things). This would be beneficial for OpenShift in removing the bootstrap node - but I think it would also help people test Ignition configs that are targeted for this sort of setup using any boot media (mainly including our cloud instances).

I went to look at implementing this based on our current code for the LiveISO and realized that we can't really do this today from Ignition primarily because of the same /etc issue - we need /etc to be set up before we even transition into the real root.

Interaction with Ignition

Change ignition-firstboot-complete.service to have ConditionKernelCommandLine=!coreos.volatile so that Ignition reruns each boot.

Transitioning to stateful

For the OpenShift case we want to support transitioning from volatile to stateful. That'd just be rpm-ostree kargs --delete=coreos.volatile. Anything that the user wants to preserve here (including e.g. a new Ignition config) would be explicitly stored in /boot.

Alternative: ask systemd to reconsider their implementation, or tweak the implementation to behave differently on coreos

This seems possible, and would benefit more than just us - I doubt many people are using systemd.volatile today for similar reasons as us.

[cgwalters]

The /etc issue in this setup closely mirrors ostreedev/ostree#2113