Implement B/G for service mesh providers

[stefanprodan]

This PR implements Blue/Green deployments for all service mesh providers. Flagger will use the B/G strategy when canaryAnalysis.iterations is specified in the canary spec instead of maxWeight/stepWeight. After the analysis finishes, the traffic is routed to the canary (green) before triggering the primary (blue) rolling update, this ensures a smooth transition to the new version avoiding dropping in-flight requests during the Kubernetes deployment rollout.

Blue/Green steps:

• scale up green
• run conformance tests on green
• run load tests and metric checks on green
• route traffic to green
• promote green spec over blue
• wait for blue rollout
• route traffic to blue
• scale down green

Example:

apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
  name: podinfo
  namespace: test-istio
spec:
  provider: istio
  targetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: podinfo
  autoscalerRef:
    apiVersion: autoscaling/v2beta1
    kind: HorizontalPodAutoscaler
    name: podinfo
  service:
    port: 9898
    gateways:
    - public-gateway.istio-system.svc.cluster.local
    - mesh
    hosts:
    - podinfo.istio.flagger.dev
    trafficPolicy:
      tls:
        mode: DISABLE
  canaryAnalysis:
    # schedule interval
    interval: 60s
    # iterations to be run for the B/G analysis
    iterations: 10
    # max number of failed metric checks before rollback
    threshold: 10
    metrics:
    - name: request-success-rate
      threshold: 99
      interval: 1m
    - name: request-duration
      threshold: 500
      interval: 30s
    webhooks:
      - name: acceptance-test
        type: pre-rollout
        url: http://flagger-loadtester.test/
        timeout: 30s
        metadata:
          type: bash
          cmd: "curl -sd 'test' http://podinfo-canary:9898/token | grep token"
      - name: load-test
        url: http://flagger-loadtester.test/
        timeout: 5s
        metadata:
          type: cmd
          cmd: "hey -z 1m -q 10 -c 2 http://podinfo:9898/"

Flagger image: weaveworks/flagger:service-mesh-blue-green-9d856a4

Fix: #294