StartContainer behavior not matching gNOI specification

[f-sommerauer]

Per this:

  // Start returns an error. A started container is identified by an instance
  // name, which  can optionally be supplied by the caller otherwise the target
  // should provide one. When the instance name already exists, the target
  // should return an error if the container is running or if it is stopped but
  // the request has more fields than just the instance name. If the container
  // is stopped and the request contains only the instance name then Start
  // should start the container.

Currently calling the StartContainer RPC with just the instance_name (to start a previously stopped container) will result in an error instead of starting the container.