add a fallback for default configuration when fqdn is unknown

[robbkidd]

Sometimes ohai is unable to determine a host's FQDN resulting in a nil node['fqdn']. This makes the current default fail during reconfigure. The supermarket-omnibus-cookbook has been working around it in most cases by placing an entry in /etc/hosts for the host's hostname if needed. We should consider handling a fallback here within the internal omnibus cookbook, e.g. for folks who might be hit this by using chef-ingredient directly.