Implement canary service target port

[stefanprodan]

This PR adds the service.targetPort to the canary CRD. The target port can be specified using the container port number or port name.

Example:

apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
  name: podinfo
spec:
  service:
    # ClusterIP port number
    port: 80
    # ClusterIP port name can be http or grpc (default http)
    portName: http
    # container port number or name
    targetPort: 9898
    # add all the other container ports
    # when generating ClusterIP services (default false)
    portDiscovery: true

Automated end-to-end tests coverage:

• Kubernetes CNI target port number
• Kubernetes CNI target port name
• Istio target port number
• Istio target port name
• Linkerd target port number
• Linkerd target port name
• NGINX target port number
• NGINX target port name
• Gloo target port number
• Gloo target port name

Fix: #279
Fix: #328