Set up auto update for image tag in manifests

[terrytangyuan]

Some context: #88 (comment)

The XGBoost operator's manifests in kubeflow/manifests should be automatically updated with newly built images in GCP.

Reference: https://github.com/kubeflow/testing/blob/master/apps-cd/applications.yaml

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[pingsutw]

/assign

[pingsutw]

@terrytangyuan Could we close it?
I don't know how to close it. I don't see the close button in Github