postprocess: Downgrade warning about files in /var to print

It's really just informative...
Switch to taking JSON as input, generate "treefile" from products.json

This is somewhat similar to what we've been doing with Continuous; we
take the manifest.json, and turn it into a "snapshot".  Except here
there is a notion of inheritance.

This gets stored into the tree as /usr/share/rpm-ostree/treefile.json.

Additionally, it goes into the autobuilder directory in
products-built.json.  Though really we should split up that file,
since it will be kind of...large.
fedostree/products: Add comments to trees