fix(*): do a wholesale rename of the dbus interfaces

Replace the org.chromium namespace with com.coreos.update1. This is part
of making it possible to document the "update ready" interface.

Along the way this commit also removes a number of interfaces that we
don't plan to support such as setting channel and track over dbus.
chore(main): Chrome OS -> CoreOS
Merge pull request #26 from philips/rename-update-engine-dbus-entries

fix(*): do a wholesale rename of the dbus interfaces
chore(systemd): delete update-engine-reboot-manager
fix(systemd): remove unused PathExists from unit files