Implement confirm rollout gate, hook and API

[stefanprodan]

This PR adds the confirm-rollout webhook type. The confirmation hooks are executed before the pre-rollout hooks. Flagger will halt the canary traffic shifting and analysis until the confirm webhook returns HTTP status 200.

Manual gating:

  canaryAnalysis:
    webhooks:
      - name: "start gate"
        type: confirm-rollout
        url: http://flagger-loadtester.test/gate/halt

The /gate/halt returns HTTP 403 thus blocking the rollout. If you have notifications enabled, Flagger will post a message to Slack or MS Teams if a canary rollout is waiting for approval.

Change the URL to /gate/approve to start the canary analysis:

  canaryAnalysis:
    webhooks:
      - name: "start gate"
        type: confirm-rollout
        url: http://flagger-loadtester.test/gate/approve

Manual gating API:

Set the confirmation URL to /gate/check:

  canaryAnalysis:
    webhooks:
      - name: "ask confirmation"
        type: confirm-rollout
        url: http://flagger-loadtester.test/gate/check

By default the gate is closed, you can start or resume the canary rollout with:

kubectl -n test exec -it flagger-loadtester-xxxx-xxxx sh

curl -d '{"name": "podinfo","namespace":"test"}' http://localhost:8080/gate/open 

Pause rollout:

curl -d '{"name": "podinfo","namespace":"test"}' http://localhost:8080/gate/close 

Note that in order to tests this PR you have to update the Canary CRD and use the following images:

• weaveworks/flagger:gates-f204fe5
• weaveworks/flagger-loadtester:0.6.1

Fix: #163

[mumoshu]

LGTM! Thanks a lot for your effort ☺️

[aackerman]

This is awesome @stefanprodan, thanks for all the work on this!