rpm-ostree: deregister as rpm-ostree driver when terminating

[kelvinfan001]

Currently, if Zincati was ever active on a machine, it'll remain as the "updates driver" for rpm-ostree even if an admin explicitly tries to disable Zincati by stopping zincati.service.
After coreos/rpm-ostree#2749 is implemented on the rpm-ostree side, Zincati should deregister itself when a user does e.g. systemctl stop zincati.service, possibly by handling the SIGTERM signal or adding an ExecStop= in the systemd unit.

[kelvinfan001]

rpm-ostree has opted to not have a deregister API; instead, 'inactive' update drivers are ignored and achieves essentially the same purposes as deregistering.