Fix cookbook crash if a node is missing a hostname

There are cases where ohai cannot determine a fqdn for a node (like if
dns is not working) and node['hostname'] can be nil.  This deals with
that situation by returning the node name in place of the hostname if
node['hostname'] is nil.

Change-Id: Ibfab60c42a482604a25d5ae7353a9a70b257360d
Merge pull request #244 from scottso/fix_missing_hostname_crash

Fix cookbook crash if a node is missing a hostname