Allow apply blocks in plans to contain node definitions

Currently, Bolt does not support evaluating node definitions when running plans or applying Puppet code. Accordingly, Puppet does not allow node definitions to be specified in plans. In order to now support node definitions with Bolt, we need to relax the restriction in Puppet so the node definitions can be loaded and used by Bolt.

For more information: https://github.com/puppetlabs/bolt/issues/1338