handle case where Eucalyptus metadata API returns 404

Eucalyptus 3.3.0 and earlier has http://169.254.169.254/ return a 404.  This will make Ohai work with eucalyptus again like it did in Ohai 6.16.0 and earlier.

Note:  This bug is fixed in a future (unreleased) version of eucalyptus, probably 3.3.1.