Add HTTP match and rewrite to Canary service spec

[stefanprodan]

Add Istio HTTPMatchRequest and HTTPRewrite to Canary CRD.

apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
  name: podinfo
spec:
  service:
    port: 9898
    match:
      - uri:
          prefix: /podinfo/
    rewrite:
      uri: /
.........

Test image: quay.io/stefanprodan/flagger:http-match-eabef3d

Fix: #50

[codecov-io]

Codecov Report

Merging #55 into master will increase coverage by 0.19%.
The diff coverage is 93.1%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   56.69%   56.88%   +0.19%     
==========================================
  Files           9        9              
  Lines        1344     1350       +6     
==========================================
+ Hits          762      768       +6     
  Misses        448      448              
  Partials      134      134

Impacted Files	Coverage Δ
pkg/controller/router.go	85.37% <93.1%> (+0.42%)	⬆️

---

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 c835892...eabef3d. Read the comment docs.