Upgrade collectd module

pdxcat/collectd haven't released a new version with updates to the
varnish plugin that we need so we have forked it into alphagov.
Add varnish collectd plugin to frontend boxes

This plugin collects the following stats from varnishstat

CollectCache
CollectBackend
CollectConnections
CollectSHM
CollectFetch
CollectTotals
CollectWorkers
CollectCache
Merge pull request #363 from alphagov/collect-varnish-stats

Collect varnish stats