varnishstat defaultBinary variable is not used

[joshuaspence]

I was trying to use the varnish input but was getting the following error:

> telegraf -config /etc/telegraf/telegraf.conf -input-filter varnish -test
* Plugin: varnish, Collection 1
2016/09/11 22:54:57 error gathering metrics: error running varnishstat: fork/exec : no such file or directory

It took me a while to figure it out, but it seems that I am required to specified the path to the varnishstat binary. This wasn't obvious from the usage text, which seemed to imply that the default should be set:

> telegraf -usage varnish

# A plugin to collect stats from Varnish HTTP Cache
[[inputs.varnish]]
  ## The default location of the varnishstat binary can be overridden with:
  binary = "/usr/bin/varnishstat"

  ## By default, telegraf gather stats for 3 metric points.
  ## Setting stats will override the defaults shown below.
  ## Glob matching can be used, ie, stats = ["MAIN.*"]
  ## stats may also be set to ["*"], which will collect all stats
  stats = ["MAIN.cache_hit", "MAIN.cache_miss", "MAIN.uptime"]

Looking at the source code, it looks like defaultBinary is set to /usr/bin/varnishstat but never referenced.