docker_image: add support for pulling an image from a registry

This allows the user to omit the `path` argument from the module with `state=present` and the module will attempt to pull the image from a Docker registry instead of building it from source. Even though the `docker` module will already attempt to pull an image when it needs it to run a container, being able to ensure an image already exists on a set of machines is helpful when instances on different hosts need to be created at roughly the same time.

I also changed the `build` option for the `state` module parameter to `force` to better match the semantics of what it would do when building or when pulling.