I'm sad that some of these debug messages aren't louder errors, but yay Ohai 7.

Consider these edits @jaymzh 

```
begin
  fqdn_output = from("hostname --fqdn")

  if fqdn_output.empty?
    Ohai::Log.debug("hostname --fqdn returned an empty string, retrying once.")
    fqdn_output = from("hostname --fqdn")
  end

  if fqdn_output.empty?
    Ohai::Log.debug("hostname --fqdn returned an empty string twice and will not be set.")
  else
    fqdn fqdn_output
  end

rescue Ohai::Exceptions::Exec
  Ohai::Log.debug("hostname --fqdn returned an error, probably no domain set")
end
```