fix: Set the currentStepIndex to max after bg to canary

[dthomson25]

closes #555.

@jessesuen do you know why we defaulted checkStepHashChange to return false if the currentStepHash is empty?

[codecov]

Codecov Report

Merging #558 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   85.92%   85.93%   +0.01%     
==========================================
  Files          86       86              
  Lines        8136     8136              
==========================================
+ Hits         6991     6992       +1     
+ Misses        819      818       -1     
  Partials      326      326              

Impacted Files	Coverage Δ
utils/replicaset/replicaset.go	90.03% <100.00%> (+0.34%)	⬆️

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[jessesuen]

What happens if someone does: Canary -> BlueGreen -> Canary?

Do we null out the status.canary fields when moving away from canary? Otherwise we might carry over defunct stephash / stepindex information and still have this problem

[dthomson25]

The controller nulled all the canary status field when the user switches the Rollout from canary back to bluegreen