Add CircleCI workflow for end-to-end testing with Kubernetes Kind

[stefanprodan]

The e2e testing infrastructure is powered by CircleCI and Kubernetes Kind.

CircleCI e2e workflow:

• install latest stable kubectl
• build Kubernetes Kind from master
• create local Kubernetes cluster with kind
• install latest stable Helm CLI
• deploy Tiller on the local cluster
• install Istio CRDs with Helm
• install Istio control plane and Prometheus with Helm
• build Flagger container image
• load Flagger image onto the local cluster
• deploy Flagger in the istio-system namespace
• create a test namespace with Istio injection enabled
• deploy the load tester in the test namespace
• deploy a demo workload (podinfo) in the test namespace
• test the canary initialization
• test the canary analysis and promotion

Fix: #52

[codecov-io]

Codecov Report

Merging #53 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
=======================================
  Coverage   56.34%   56.34%           
=======================================
  Files           9        9           
  Lines        1340     1340           
=======================================
  Hits          755      755           
  Misses        452      452           
  Partials      133      133

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