metadata: expose instance type/size

This is a feature request for the link-local `/metadata/v1` endpoint (or a documentation enhancement, if the feature is already in place but not documented).

It would be nice to have an "instance type" field as part of the node metadata, so that an host could introspect and detect its own size at runtime. Regarding the content of this additional field, matching the instance type "slug" that is used for the creation API would be good.

We are currently adding introspection logic for CoreOS-style systems, and from an initial survey DO is the only major environment not exposing this piece of information to nodes: https://github.com/coreos/afterburn/issues/277.

/cc @hilary @snormore for further internal triaging/dispatching