Collect varnish stats

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 and collect;
CollectCache
CollectBackend
CollectConnections
CollectSHM
CollectFetch
CollectTotals
CollectWorkers
CollectCache