Re-enable varnishkafka delivery error alerts

This re-enables the icinga alerts for varnishkafka delivery errors that
was disabled in https://gerrit.wikimedia.org/r/#/c/138302/1.  The
alert thresholds have been adjusted, and Kafka has been mostly good
since a cluster reinstall on June 24th.

Change-Id: Ia7b206c429a50678c46e30c2d93ccd835f5eff62