Kubernetes support

[erikh]

I think this will require a rearchitecture of the volplugin.

I believe these interfaces are the key: https://gist.github.com/erikh/53c5b5d502d7f9f19932f20aa8803a61

This will allow volplugin to act as a high-level driver, providing high-level communication between docker/k8s and volplugin but not doing any direct work itself. This will be provided by the api library.

Since the apiserver's code for many things is ridiculously similar, we're also going to condense that. It will also implement these interfaces along with a docker and k8s version.

Methods that are not shared do not live in the interfaces, f.e. docker.Capabilities is a safe bet. I don't think in practice that this'll be terrible; almost all of the responses are static and don't even depend on any state.