Adding selectable inference for automl generated model

1. Added attach() method to AutoML to allow attaching an AutoML object
to an existing AutoMLJob
2. Added create_model() method to AutoML that returns a PipelineModel
that can be used by the
3. Added a util function validate_and_update_inference_response() to
update the inference container list based on the requested
inference response keys
4. Updated and added unit tests

Author: pialidas