Should we persist with parsing JSON config file for Firecracker VM opts?

PRs #109 and #105 provide clients the ability to pass Firecracker VM options using the `FirecrackerConfig` protobuf message. As per https://github.com/firecracker-microvm/firecracker-containerd/pull/109#discussion_r259130140:

> Originally JSON was added as temp solution to simplify running. Here we have to manage two entities for configuration and keep `overrideVMConfigFromTaskOpts` to overwrite config params. This makes it more complicated than it should be and I see no reasons to keep old config.

Let's discuss the tradeoffs for continuing to parse default values from JSON config. So, option A is continue doing what's being done today (`read json + build FC config + override protobuf`). Option B would be `defaults + protobuf`.