Use SyslogIdentifier to improve logging

Reasoning is the same as for matrix-org/synapse#5023.

For us, the journal used to contain `docker` for all services, which
is not very helpful when looking at them all together (`journalctl -f`).

roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2

@@ -43,6 +43,7 @@ ExecReload=/usr/bin/docker exec matrix-coturn kill -USR2 1
 
 Restart=always
 RestartSec=30
+SyslogIdentifier=matrix-coturn
 
 [Install]
 WantedBy=multi-user.target