**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
"""
    Sets a version in the model.
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:MLEngineSetDefaultVersionOperator`
    The model should be specified by `model_name` to be the default. The name of the version should be
    specified in the `version_name` parameter.
    :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. (templated)
    :type version_name: str
    :param project_id: The Google Cloud project name to which MLEngine model belongs.
        If set to None or missing, the default project_id from the Google Cloud connection is used.
        (templated)
    :type project_id: str
    :param gcp_conn_id: The connection ID to use when fetching connection info.
    :type gcp_conn_id: str
    :param delegate_to: The account to impersonate using domain-wide delegation of authority,
        if any. For this to work, the service account making the request must have
        domain-wide delegation enabled.
    :type delegate_to: str
    :param impersonation_chain: Optional service account to impersonate using short-term
        credentials, or chained list of accounts required to get the access_token
        of the last account in the list, which will be impersonated in the request.
        If set as a string, the account must grant the originating account
        the Service Account Token Creator IAM role.
        If set as a sequence, the identities from the list must grant
        Service Account Token Creator IAM role to the directly preceding identity, with first