recover loglevel for ExecStart in systemd config (#4023)

* it was lost in d54eb6d
* "systemctl start celery" doesn't respect CELERYD_LOG_LEVEL as
  "systemctl restart celery" does
* give options in the same order as ExecReload for symmetry

extra/systemd/celery.service

@@ -9,8 +9,8 @@ Group=celery
 EnvironmentFile=-/etc/conf.d/celery
 WorkingDirectory=/opt/celery
 ExecStart=/bin/sh -c '${CELERY_BIN} multi start $CELERYD_NODES \
-	-A $CELERY_APP --logfile=${CELERYD_LOG_FILE} \
+	-A $CELERY_APP --pidfile=${CELERYD_PID_FILE} --logfile=${CELERYD_LOG_FILE} \
-	--pidfile=${CELERYD_PID_FILE} $CELERYD_OPTS'
+	--loglevel="${CELERYD_LOG_LEVEL}" $CELERYD_OPTS'
 ExecStop=/bin/sh -c '${CELERY_BIN} multi stopwait $CELERYD_NODES \
 	--pidfile=${CELERYD_PID_FILE}'
 ExecReload=/bin/sh -c '${CELERY_BIN} multi restart $CELERYD_NODES \