feat: default to MCR for Kubernetes images

[mboersma]

Reason for Change:
Switches the default image base for Kubernetes components from k8s.gcr.io to mcr.microsoft.com.

This change was coming, but the switching of base URLs for new cluster-autoscaler releases has made it a priority.

Issue Fixed:
Refs #3024

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3046 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3046   +/-   ##
=======================================
  Coverage   70.62%   70.62%           
=======================================
  Files         145      145           
  Lines       25147    25147           
=======================================
  Hits        17761    17761           
  Misses       6283     6283           
  Partials     1103     1103           

Impacted Files	Coverage Δ
pkg/api/addons.go	97.74% <100.00%> (ø)
pkg/api/components.go	96.89% <100.00%> (ø)
pkg/api/defaults.go	92.20% <100.00%> (ø)

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[jackfrancis]

/lgtm

[mboersma]

For more background on what nudged AKS Engine to make this change, see Kubernetes vanity domain flip.