**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
#
#
import copy
import datetime
import unittest
from unittest.mock import ANY, MagicMock, patch
import httplib2
from googleapiclient.errors import HttpError
from airflow.exceptions import AirflowException
from airflow.models import TaskInstance
from airflow.models.dag import DAG
from airflow.providers.google.cloud.operators.mlengine import (
    AIPlatformConsoleLink,
    MLEngineCreateModelOperator,
    MLEngineCreateVersionOperator,