breaking: change Airflow model_config order; make instance_type optional in model_config and Model.prepare_container_def

[laurenyu]

Issue #, if available:

Description of changes:
This is motivated by the Airflow model_config method unnecessarily requiring instance_type even though the parameter is unused. The reason prepare_container_def might use instance_type is because for Framework models, the instance type is used to pick "cpu" or "gpu" for the image tag if the tag is not provided. However, the base class's function need not require the argument.

Due to the maze of inheritance, I chose not to remove the arg entirely. This is because the various _create_sagemaker_model() calls would need to be modified (and perhaps re-implemented in specific subclasses) so that self.image is populated before calling self.prepare_container_def.

The other improvement that can be saved for a separate PR is to make instance_type optional in each framework's implementation of prepare_container_def if self.image is not None.

Testing done:
unit tests

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[laurenyu]

closing now, and will redo after #1567