fix: remove old workflows on addon checksum change

[ccpeng]

fixes: #285 #174

with contribution from @estela-ramirez

a new addon (but has the same checksum from recent past) won't generate a new workflow since a previous workflow was already created; as a result, the addon gets stuck on pending

fix is to purge old workflows so a new workflow can get created

[codecov[bot]]

Codecov Report

Merging #297 (9ebe761) into master (ef27eb5) will decrease coverage by 0.39%. The diff coverage is 64.28%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   56.20%   55.81%   -0.39%     
==========================================
  Files          14       14              
  Lines        1893     1933      +40     
==========================================
+ Hits         1064     1079      +15     
- Misses        715      734      +19     
- Partials      114      120       +6     

Files	Coverage Δ
pkg/workflows/workflow.go	68.71% <100.00%> (+0.09%)	:arrow_up:
controllers/addon_controller.go	53.53% <63.41%> (-1.72%)	:arrow_down:

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[kevdowney]

I think the Addon can still get stuck in pending if there's no further updates or controllers aren't updating... we can additionally add a check before return from https://github.com/keikoproj/addon-manager/blob/ef27eb55c2f360f9aa6d24d4ca60c0117bdb79bf/controllers/addon_controller.go#L350

if instance.IsPending() {
  return ctrl.Result{RequeAfter: 5 * Duration.Seconds}, nil
}