**Description**

The operator is arguably unnecessarily limited to AI Platform’s standard images. The only change that is required to lift this constraint is making `package_uris` and `training_python_module` optional with default values `[]` and `None`, respectively. Then, using `master_config`, one can supply `imageUri` and run any image of choice.

**Use case / motivation**

This will open up for running arbitrary images on AI Platform.

**Are you willing to submit a PR?**

If the above sounds reasonable, I can open pull requests.
airflow/providers/google/cloud/operators/mlengine.py
MLEngineListVersionsOperator, MLEngineDeleteVersionOperator.
    :param model_name: The name of the Google Cloud ML Engine model that the version
        belongs to. (templated)
    :type model_name: str
    :param version_name: A name to use for the version being operated upon.
        If not None and the `version` argument is None or does not have a value for
        the `name` key, then this will be populated in the payload for the
        `name` key. (templated)
    :type version_name: str
    :param version: A dictionary containing the information about the version.
        If the `operation` is `create`, `version` should contain all the
        information about this version such as name, and deploymentUrl.
        If the `operation` is `get` or `delete`, the `version` parameter
        should contain the `name` of the version.
        If it is None, the only `operation` possible would be `list`. (templated)
    :type version: dict
    :param operation: The operation to perform. Available operations are:
        *   ``create``: Creates a new version in the model specified by `model_name`,
            in which case the `version` parameter should contain all the
            information to create that version
            (e.g. `name`, `deploymentUrl`).
        *   ``set_defaults``: Sets a version in the model specified by `model_name` to be the default.
            The name of the version should be specified in the `version`
            parameter.
        *   ``list``: Lists all available versions of the model specified
            by `model_name`.
        *   ``delete``: Deletes the version specified in `version` parameter from the
            model specified by `model_name`).
            The name of the version should be specified in the `version`
            parameter.