istio: Add source labels to analysis matching rules

[stefanprodan]

This PR adds the sourceLabels filed to the Canary CRD. This allows running an A/B test using source labels matches when a service is attached to the Istio mesh gateway.

Example:

kind: Canary
metadata:
  name: backend
spec:
  provider: istio
  service:
    port: 9898
    gateways:
      - mesh
  analysis:
    interval: 15s
    threshold: 10
    iterations: 10
    match:
    - sourceLabels:
        app: frontend

Fix: #580

For testing update the CRD with kubectl apply -f https://raw.githubusercontent.com/weaveworks/flagger/istio-source-labels/artifacts/flagger/crd.yaml before using sourceLabels.

[codecov-io]

Codecov Report

Merging #594 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #594   +/-   ##
=======================================
  Coverage   55.00%   55.00%           
=======================================
  Files          62       62           
  Lines        5303     5303           
=======================================
  Hits         2917     2917           
  Misses       1959     1959           
  Partials      427      427           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f5a3b9d...4f299e5. Read the comment docs.