fix: override components command config during upgrade

[jackfrancis]

Reason for Change:

This PR enforces component command configuration override during upgrade, which is documented here:

https://github.com/Azure/aks-engine/blob/master/docs/topics/clusterdefinitions.md#components

Specifically:

this configurable vector is designed for cluster creation only. Using aks-engine upgrade on a cluster will override the original, user-configured settings during the upgrade operation, rendering an upgraded cluster with the AKS Engine defaults for kube-controller-manager, cloud-controller-manager, kube-apiserver, kube-scheduler, and kube-addon-manager.

We would be true to the above documentation to statically override all configuraiton key/vals in an upgrade flow, but in the event that we want to enable some subset of the components container configuration to be user-configurable across upgrade, this PR only enforces the config override for the "command" config, which is the only config key that our yaml manifests currently recognize.

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #2814 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2814      +/-   ##
==========================================
+ Coverage   72.35%   72.43%   +0.07%     
==========================================
  Files         137      140       +3     
  Lines       25339    25550     +211     
==========================================
+ Hits        18333    18506     +173     
- Misses       5949     5976      +27     
- Partials     1057     1068      +11     

Impacted Files	Coverage Δ
pkg/armhelpers/azurestack/httpMockClient.go	61.46% <0.00%> (-0.25%)	⬇️
pkg/armhelpers/mockclients.go	22.61% <0.00%> (-0.14%)	⬇️
pkg/armhelpers/httpMockClient.go	58.71% <0.00%> (-0.13%)	⬇️
pkg/helpers/azure_locations.go	100.00% <0.00%> (ø)	⬆️
pkg/armhelpers/subscriptions.go	60.00% <0.00%> (ø)
pkg/armhelpers/azurestack/subscriptions.go	60.00% <0.00%> (ø)
cmd/get_locations.go	87.34% <0.00%> (ø)
pkg/api/addons.go	97.61% <0.00%> (+0.01%)	⬆️
pkg/armhelpers/azurestack/azureclient.go	29.46% <0.00%> (+0.64%)	⬆️
pkg/armhelpers/azureclient.go	35.27% <0.00%> (+0.69%)	⬆️
... and 1 more

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[mboersma]

/lgtm