Update config.yaml with proper default container image

[i-chvets]

Bug Description

Currently, config.yaml refers to default Seldon Core Operator container image v1.14. As part of Kubeflow 1.8 work, new image is being used. As a result, this default configuration should be updated to use new ROCK image.

To Reproduce

Refer to corresponding config.yaml line

Environment

config.yaml for this charm.

Relevant log output

N/A