[AWS] Optional-Test-Infra Migration

[PatrickXYS]

Since I have configurated the optional-test-infra on external AWS account, and confirmed it works with basic use cases, e.g, PatrickXYS-testing/flux-testing#4

And I think we can migrate some of kubeflow repos to external optional-test-infra, including:

repo list

kubeflow/manifests

kubeflow/kfctl
kubeflow/katib
kubeflow/kubeflow
kubeflow/kfserving
kubeflow/tf-operator
kubeflow/pytorch-operator
kubeflow/xgboost-operator

The workflow will be:

1. Configure organization webhook for external optional-test-infra
2. Configure plugins/trigger to repo-level, and configure config with each repo's presubmit
3. Configure each of repos one by one to avoid test failure

Note: We haven't reached to FullCD yet since public ECR support is on the way.

Result: testing will act exactly as previous, but all the resources will be consumed from external OSS account.

/cc @Bobgy @andreyvelich @theofpa

[PatrickXYS]

/cc @Jeffwan

[PatrickXYS]

Migration Finish List:

pytorch-operator: kubeflow/pytorch-operator#316
kfctl: kubeflow/kfctl#482
manifests: kubeflow/manifests#1726
kubeflow: #878
xgboost-operator: #879
katib: kubeflow/katib#1423
kfserving: kserve/kserve#1329
tf-operator: #892