Implement directory environments

This series of commits moves around the site.pp file to manifests/site.pp.  It also removes the section of the master repository class that would add an environment-per-student stanza to the puppet.conf.  It then configures the master's puppet.conf so that it will use directory environments.

This removes the deprecation warnings that we've been seeing since we updated to PE 3.3.

Happily, the classroom environment was pretty much already set for directory environments.