Do not kill reprepro when stopping a time-based APT snapshot

By default, Systemd will also send KillSignal to every process in the same
control-group of the main service process. Because we want to minimize the risk
of interrupting reprepro before it finishes, let's set KillMode=process to
avoid sending any signal to it before TimeoutStopSec has passed.

refs sysadmin#17895