Implement port discovery

[stefanprodan]

This PR adds port discovery boolean field to the canary service spec. If port discovery is enabled, Flagger scans the deployment pod template and extracts the container ports excluding the port specified in the canary service spec and Istio/App Mesh proxy ports. All the extra ports will be used when generating the ClusterIP services.

For a deployment that exposes two ports e.g.:

apiVersion: apps/v1
kind: Deployment
spec:
  template:
    metadata:
      annotations:
        prometheus.io/scrape: "true"
        prometheus.io/port: "9899"
    spec:
      containers:
      - name: app
        ports:
        - containerPort: 8080
          name: http
          protocol: TCP
        - containerPort: 9899
          name: prom-metrics
          protocol: TCP

You can enable port discovery so that Prometheus will be able to reach port 9899 over mTLS:

apiVersion: flagger.app/v1alpha3
kind: Canary
spec:
  service:
    # container port used for canary analysis
    port: 8080
    # port name can be http or grpc (default http)
    portName: http
    # add all the other container ports
    # when generating ClusterIP services (default false)
    portDiscovery: true
    trafficPolicy:
      tls:
        mode: ISTIO_MUTUAL

Fix: #171

[marcoferrer]

Excited to see this getting attention.