Implement helm and bash pre-rollout hooks

[stefanprodan]

This PR adds two blocking commands to Flagger's test runner:

• type: helm intended for running helm tests
• type: bash intended for running bats tests

Usage

Deploy the helm test runner in the kube-system namespace using the tiller service account:

kubectl apply -f artifacts/helmtester

Run helm and bats tests before the canary starts receiving traffic:

  canaryAnalysis:
    webhooks:
      - name: "helm tests"
        type: pre-rollout
        url: http://flagger-helmtester.kube-system/
        timeout: 3m
        metadata:
          type: "helm"
          cmd: "test podinfo --cleanup"
      - name: "bats tests"
        type: pre-rollout
        url: http://flagger-batstester.istio-system/
        timeout: 5m
        metadata:
          type: "bash"
          cmd: "bats /tests/integration-test.bats"

Note that you should create a ConfigMap with your bats tests and mount it inside the tester container.