Add option to skip the canary analysis

[stefanprodan]

In emergency cases, you may want to skip the analysis phase and ship changes directly to production. At any time you can set the spec.skipAnalysis: true. When skip analysis is enabled, Flagger checks if the canary deployment is healthy and promotes it without analysing it. If an analysis is underway, Flagger cancels it and runs the promotion.

Fix: #45

[codecov-io]

Codecov Report

Merging #46 into master will increase coverage by 0.01%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   56.62%   56.63%   +0.01%     
==========================================
  Files           9        9              
  Lines        1305     1333      +28     
==========================================
+ Hits          739      755      +16     
- Misses        438      446       +8     
- Partials      128      132       +4

Impacted Files	Coverage Δ
pkg/controller/scheduler.go	41.63% <57.14%> (+1.89%)	⬆️

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