Add puppetserver_trusted_agents parameter (#528)

I'm using octocatalog-diff to fetch catalogs from a puppetserver.
By default, agents can only fetch their own catalogs. I need to
reconfigure the puppetserver so that octocatalog-diff can fetch
*any* catalog. This commit adds a new
server_puppetserver_trusted_agents parameter that makes this possible.

See.
https://github.com/github/octocatalog-diff/blob/master/doc/advanced-puppet-master.md#certificate-authorization