**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
TRAINING_DEFAULT_ARGS = {
        'project_id': 'test-project',
        'job_id': 'test_training',
        'package_uris': ['gs://some-bucket/package1'],
        'training_python_module': 'trainer',
        'training_args': '--some_arg=\'aaa\'',
        'region': 'us-east1',
        'scale_tier': 'STANDARD_1',
        'labels': {'some': 'labels'},
        'task_id': 'test-training',
        'start_date': days_ago(1),
    }
    TRAINING_INPUT = {
        'jobId': 'test_training',
        'labels': {'some': 'labels'},
        'trainingInput': {
            'scaleTier': 'STANDARD_1',
            'packageUris': ['gs://some-bucket/package1'],
            'pythonModule': 'trainer',
            'args': '--some_arg=\'aaa\'',
            'region': 'us-east1',
        },
    }
    def setUp(self):