Make it harder to rpm-ostree upgrade when using an update driver

[jlebon]

Follow-up to #1747.

In FCOS, it's too easy for users to type rpm-ostree upgrade and blow past update barriers (see e.g. coreos/fedora-coreos-tracker#731 (comment)).

One path is to make FCOS use checksum origins like RHCOS, though that then drives knowledge about the OSTree refspec into Zincati.

Another approach which I think would work well with #1747 is to have rpm-ostree upgrade print out a message if an update driver is registered, and require some override switch. (Maybe even down the line have it call out to the update driver which could check for updates immediately?)

[jlebon]

Something like:

$ rpm-ostree upgrade
This node is updated by $driver_pretty_name; see the documentation
for that software at $driver_systemd_unit_documentation. Use --force
to force upgrading anyway.

[cgwalters]

100% on board with that!

[travier]

Sounds great!