Setting log_statistics_interval to 15 for varnishkafka

This ensures that metrics in gmond are fresh whenever gmetad polls.
This isn't explicitly necessary now that the custom vanrishkafka
ganglia python module is computing the per_second rates on its own
i.e. there are no positively sloped metrics, but it seems nice to
be fresh when gmetad asks.

Change-Id: I17f0aaa887ca8a4a4d673c98e47a0819b078c84a