Datasources for categories

[JonKohler]

From @ranman166 on #217

We are looking to be able to reference categories without having them managed as resources.

For instance, we want to be able to create a bunch of category keys and later have users add values if needed and reference them within their vm creation without having users create or destroy the keys.

A win would be referencing them without having to deal with the "id" within the vm resource. We could do this through local variables in a module, but managing their "values" will get out of hand.