allow dlcs-varnish to run when the INCOMING_QUEUE is not set

[JackLewis-digirati]

Based on this comment:

One thought - it might be good to allow this to work if INCOMING_QUEUE is missing. E.g. if we scale out for 4hours to cope with a known load we might want additional varnish instances running without worrying about bans.

Currently, the varnish cleanup handler requires an INCOMING_QUEUE to be set, meaning that the application will fail to run if the cleanup handler isn't correctly set up. This could be an issue if we want to run the varnish cache separately from the cleaner. This ticket is to make it so that these applications can be run separately