fix dns_discovery init() call breaks in windows

https://github.com/hudl/fargo/blob/master/dns_discover.go#L75

This line gets executed when the fargo package are imported, and on a windows system, this file doesn't exist, and therefore breaks the application even if the application does not really use the dns discovery feature.

So I basically change the init() call to a normal function and propagate the error so it doesn't break on windows when the feature is not enabled.