OHAI-339 Unable to detect IPAddress on CoreOS/Gentoo

On Gentoo based distributions the `ip` executable is not linked to `/sbin/ip` but to `/usr/bin/ip`. This fixes #397 Check for both locations. 

A general fix could be:
```
ENV['PATH'].split(File::PATH_SEPARATOR).any? do |directory|
    File.executable?(File.join(directory, "ip"))
  end
```

But maybe overkill. So, let's keep it simple. :)
Doesn't #iproute2_binary_available? just return true/false? This seems like a convoluted way to ensure we're testing the right code portion.