Send worker-sender celery logs to /dev/null

We are using our custom logger to log to `NOTIFY_LOG_PATH`, so this
logging from celery is neither needed nor desired.

We also need to define the location of the pidfiles, because of what
appears to be a bug in celery where it uses the location of logs to
infer the location of the pidfiles if it is not defined, i.e. in this
case it was trying to find the pidfiles in `/dev/null/%N.pid`.

scripts/paas_app_wrapper.sh

@@ -22,7 +22,7 @@ case $NOTIFY_APP_NAME in
     ;;
   delivery-worker-sender)
     exec scripts/run_multi_worker_app_paas.sh celery multi start 3 -c 10 -A run_celery.notify_celery --loglevel=INFO \
-    -Q send-sms-tasks,send-email-tasks
+    --logfile=/dev/null --pidfile=/tmp/celery%N.pid -Q send-sms-tasks,send-email-tasks
     ;;
   delivery-worker-periodic)
     exec scripts/run_app_paas.sh celery -A run_celery.notify_celery worker --loglevel=INFO --concurrency=2 \