Fix cleanup Prow job and setup alert

[chizhg]

1. github.com/google/go-containerregistry needs to be updated to the latest version. It's now using a very old one.
2. gcloud auth configure-docker command is needed for running the cleanup tool
3. Configure the cleanup Prow job to send alerts to Slack if it fails

/cc @upodroid @kvmware

[upodroid]

This is not needed. We already authenticate to gcr.io

test-infra/images/prow-tests/Dockerfile

Line 242 in 23df246

RUN docker-credential-gcr configure-docker --registries=gcr.io,us-docker.pkg.dev

Unless we are logging in to other registries.

[chizhg]

This is not needed. We already authenticate to gcr.io

test-infra/images/prow-tests/Dockerfile

Line 242 in 23df246

RUN docker-credential-gcr configure-docker --registries=gcr.io,us-docker.pkg.dev

Unless we are logging in to other registries.

I did some more tests, and have confirmed we need two changes together to make it working:

1. github.com/google/go-containerregistry dep version needs to be updated. It's now using a very old version.
2. gcloud auth configure-docker needs to be run after gcloud auth activate-service-account.

I'm not entirely sure about the reason, but I have tested it at https://prow.knative.dev/view/gs/knative-prow/logs/ci-knative-cleanup-chizhg-test/1509776600835559424 and it's working.

I have updated the PR description as well.

[chizhg]

@upodroid can I get an LGTM if this looks good to you? Thanks!

[upodroid]

LGTM, large diff though

[knative-prow]

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[chizhg]

Yeah, mostly vendor/ updates though. There are some discussions in knative/infra#134 about it.

[knative-prow]

@chizhg: Updated the job-config configmap in namespace default at cluster default using the following files:

• key test-infra.yaml using file prow/jobs/custom/test-infra.yaml

In response to this:

1. github.com/google/go-containerregistry needs to be updated to the latest version. It's now using a very old one.
2. gcloud auth configure-docker command is needed for running the cleanup tool
3. Configure the cleanup Prow job to send alerts to Slack if it fails

/cc @upodroid @kvmware

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