Automating and deploying new linux servers

Why are you not using the Nagios functionality built into Puppet? http://blog.gurski.org/index.php/2010/01/28/automatic-monitoring-with-puppet-and-nagios/

Cobbler is convoluted IMO. You could have used the Foreman to do automate most of this for you. (Including puppet cert signing). It can also do the inventory and Puppet report monitoring part. All Features:

Foreman is a Rails app so it is easily extensible. You can add code to include a post-create/destroy hook to add/remove servers in Nagios. Or simply read the Foreman hosts Db to supply lists of hosts to Nagios. The API could be helpful here.