Support for varnish instances by name

[dwradcliffe]

We have several varnish instances on one server. Each has a name and can be referenced with -n name via the command line tools.

-n  Specifies the name of the varnishd instance to get logs from. If -n is not specified, the host name is used.

(from https://www.varnish-cache.org/docs/3.0/reference/varnishstat.html)

It would be nice if the name could be specified in the yaml and the check would use it automatically. I've tried adding -n name to the bin location but that fails.

I'll probably try to add this later when I have a chance, unless someone else can do it quickly.

[clofresh]

Working on this now