Refresh systemd services on updating

[tshikaboom]

It is possible the systemd unit file (/apps/<app>/systemd.service) changes between two updates.
Currently we copy the unit file to /etc only if the service does not exist (first start). It might be a good idea to unconditionally do it and reload the unit files afterwards (systemd daemon-reload), so that we avoid errors with stale unit files if the updated container depends on stuff being set up before it starts.

A starting point for that would be to see what the create_and_start_unit() function does in update.py.