Exec so that bash is replaced by celery

This allows supervisor to send a warm shutdown to celery and wait for
celery to complete tasks and clean up before shutting down.

playbooks/roles/edxapp/templates/edx/app/edxapp/worker.sh.j2

@@ -15,4 +15,5 @@ fi
 {% set executable = edxapp_venv_bin + '/python' %}
 {% endif %}
 
-{{ executable }} {{ edxapp_code_dir }}/manage.py $@
+# We exec so that celery is the child of supervisor and can be managed properly
+exec {{ executable }} {{ edxapp_code_dir }}/manage.py $@