Add tf notebook image 1.8 version

[ankushagarwal]

This change is 

[k8s-ci-robot]

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[ankushagarwal]

/cc @lluunn

[lluunn]

Can you also update the supported versions: https://github.com/kubeflow/kubeflow/tree/master/components#tf-notebook

[lluunn]

/lgtm
/approve

[k8s-ci-robot]

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[ankushagarwal]

/retest

[ankushagarwal]

/retest