Allow blue/green deployments without a service mesh provider

[stefanprodan]

This PR adds a no-operation router that can be activated with -mesh-provider=none command flag. This allows blue/green deployments to be performed without a service mesh or ingress controller. Fix: #209

For the non service mesh promotions, the two builtin metrics are based on http_request_duration_seconds histogram that your app should expose or you can use custom promql queries like in the example below.

Blue/Green example:

apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
  name: podinfo
  namespace: test
spec:
  targetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: podinfo
  service:
    port: 9898
  canaryAnalysis:
    # schedule interval (default 60s)
    interval: 10s
    # max number of failed checks before rollback
    threshold: 10
    # number of checks to run before blue/green promotion
    iterations: 10
    # Prometheus checks
    metrics:
      - name: "tolerate 5% error rate"
        threshold: 5
        query: |
          100 - sum(
              rate(
                  http_request_duration_seconds_count{
                    namespace="test",
                    pod_name=~"podinfo-[0-9a-zA-Z]+(-[0-9a-zA-Z]+)",
                    status!~"5.*"
                  }[1m]
              )
          )
          /
          sum(
              rate(
                  http_request_duration_seconds_count{
                    namespace="test",
                    pod_name=~"podinfo-[0-9a-zA-Z]+(-[0-9a-zA-Z]+)"
                  }[1m]
              )
          ) * 100
      - name: "tolerate 500 milliseconds latency"
        threshold: 500
        interval: 1m
        query: |
          histogram_quantile(0.99,
            sum(
              rate(
                http_request_duration_seconds_bucket{
                  namespace="bg",
                  pod_name=~"podinfo-[0-9a-zA-Z]+(-[0-9a-zA-Z]+)"
                }[1m]
              )
            ) by (le)
          )
    webhooks:
      - name: smoke-test
        type: pre-rollout
        timeout: 15s
        url: https://tester.istio.weavedx.com/
        metadata:
          type: bash
          cmd: "curl -sd 'anon' http://podinfo-canary.bg:9898/token | grep token"
      - name: load-test
        url: https://flagger-loadtester.test/
        metadata:
          type: cmd
          cmd: "hey -z 1m -q 10 -c 2 http://podinfo-canary.test:9898/"