Implement ClusterSpec.data for `version:` scheme

`metadata.ClusterSpec` represents a specification for an abstract
Kubernetes cluster. For example, `version:1.7.0` represents a Kubernetes
cluster running a build from 1.7.0. This specification is primarily used
to generate ksonnet-lib.

This struct exposes a method, `data` that will retrieve the OpenAPI JSON
that specifies the API for a Kubernetes cluster. Eventually, `data` will
be able to read a file, pull from a URL, pull from a live cluster, or
pull a specific version of the API from the official Kubernetes
repository.

This commit introduces the the last of these options.