Teach `ostree-finalize-staged.service` to check for a file in `/run` to determine if it should do the finalization. This will be used in RPM-OSTree, where we want to be able to separate out "preparing updates" from "making update the default" for more fine-grained control. See: https://github.com/projectatomic/rpm-ostree/issues/1748 Closes: #1841 Approved by: cgwalters