Fixes #6134 - Discovered hosts prefix can be configured

This isn't necessary. The core test is failing due to a bug in our chained method in PuppetFactParserExtensions. Essentially, one of the interfaces has no mac in the facts, meaning `facts["macaddress_#{interface}"].try(:downcase)` is `nil`. In core, `facts[discovery_mac_fact_name].try(:downcase)` is also nil because the discovery_bootif fact is not defined. So nil == nil and we return the wrong interface, rather than falling back on the unchained method.

See https://gist.github.com/d86c495d49907b7d9fca for a possible fix. Alternatively we could skip the entire block if `facts[discovery_mac_fact_name].try(:downcase)` is `nil` since it is then not possible to match an interface.