Only running generate-varnishkafka.pyconf command if varnishkafka.stats.json exists

Hopefully this will fix some dependency issues when using varnishkafka::monitoring
on a node where varnishkafka has not yet run and created varnishkafka.stats.json.

Change-Id: Iaa546977b505a1df09ebf5ea6b45bc4e33144be0