Added virtual classroom support to the module

This simply adds two classes:

* `classroom::virtual::master`
* `classroom::virtual::agent`

For now, the students should classify their own nodes as needed. There's
significantly less automagic than in the ILT classrooms right now.

The `master` class will precreate repositories for all expected modules
on the student's master. If they'd like to add more, then can add
to the $extra_repositories parameter.

This will require @casharma to update the variable in
`classroom::params` to match the actual exercises.

Once approved, I'll bump the version and release a new version just in time for the class on Monday.