Refactor routing management

[stefanprodan]

This PR adds router.Interface with two implementations: one for Kubernetes ClusterIP services and the other for Istio virtual services.

Fix: #75

[codecov-io]

Codecov Report

Merging #72 into master will decrease coverage by 1.08%.
The diff coverage is 77.07%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   55.61%   54.53%   -1.09%     
==========================================
  Files           9       13       +4     
  Lines        1397     1533     +136     
==========================================
+ Hits          777      836      +59     
- Misses        483      557      +74     
- Partials      137      140       +3

Impacted Files	Coverage Δ
pkg/controller/controller.go	6.96% <ø> (+0.25%)	⬆️
pkg/router/factory.go	0% <0%> (ø)
pkg/controller/scheduler.go	39.56% <64.1%> (+0.23%)	⬆️
pkg/router/istio.go	80% <80%> (ø)
pkg/router/kubernetes.go	90.47% <90.47%> (ø)
pkg/loadtester/server.go	0% <0%> (ø)
pkg/loadtester/runner.go	82.97% <0%> (ø)

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