Leaf directories can output different kinds of things:
1. Platform specification. A list of components to manage.
2. Kubernetes API Objects suitable for kubectl apply -f- and friends.
3. Helm values to provide to a helm chart to render API objects.
This patch adds an output schema and a kind discriminator so the holos
cli can figure out what type of output it's working with. This makes it
possible to have a single `holos build <directory>` command that does
the right thing.
