Add extra_nodes to autosign config file for puppet

Use Case: We want to provide the ability for puppet
 to autosign nodes that arn't necessarly provisioned
 with maestro and are simply being managed staticly.

 Simply pass extra_nodes array or set hiera value
 for puppet::autosign::nodes to add the extranodes.

Change-Id: I57cc765b6e2d4ffc6aa4655e334a9a5cf30e8f3c