**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.
tests/providers/google/cloud/operators/test_mlengine.py
with self.assertRaises(AirflowException):
            MLEngineSetDefaultVersionOperator(
                task_id="task-id",
                project_id=TEST_PROJECT_ID,
                model_name=None,
                version_name=TEST_VERSION_NAME,
                gcp_conn_id=TEST_GCP_CONN_ID,
                delegate_to=TEST_DELEGATE_TO,
            )
    def test_missing_version_name(self):
        with self.assertRaises(AirflowException):
            MLEngineSetDefaultVersionOperator(
                task_id="task-id",
                project_id=TEST_PROJECT_ID,
                model_name=TEST_MODEL_NAME,
                version_name=None,
                gcp_conn_id=TEST_GCP_CONN_ID,
                delegate_to=TEST_DELEGATE_TO,
            )
class TestMLEngineListVersions(unittest.TestCase):