change service type to oneshot

[mheyen]

This patch prevents the ExecStartPost (cleanup) command to start before the backup is finished.
With the service Type=simple, the cleanup is started right after the backup has been started.
With the service Type=oneshot, the cleanup starts only when the process exited successfully.

see https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#ExecStartPre=

ExecStartPost= commands are only run after the commands specified in ExecStart= have been invoked successfully, as determined by Type= (i.e. the process has been started for Type=simple or Type=idle, the last ExecStart= process exited successfully for Type=oneshot, the initial process exited successfully for Type=forking, "READY=1" is sent for Type=notify/Type=notify-reload, or the BusName= has been taken for Type=dbus).