Update celerybeat to use sh instead of bash

Updated the init script to work without assuming bash is always available from /bin/bash/
Also see issue #2496

extra/generic-init.d/celerybeat

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh -e
 # =========================================================
 #  celerybeat - Starts the Celery periodic task scheduler.
 # =========================================================