Bump prow from v20191219-ecbeba384 to v20191219-b8438ff62

[google-oss-robot]

Included changes: kubernetes/test-infra@ecbeba3...b8438ff

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: google-oss-robot, Katharine

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[google-oss-robot]

@google-oss-robot: Updated the following 2 configmaps:

• job-config configmap in namespace default at cluster default using the following files:
  • key gcp-oss-test-infra-config.yaml using file prow/prowjobs/GoogleCloudPlatform/oss-test-infra/gcp-oss-test-infra-config.yaml
• config configmap in namespace default at cluster default using the following files:
  • key config.yaml using file prow/config.yaml

In response to this:

Included changes: kubernetes/test-infra@ecbeba3...b8438ff

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