fix: use cached v1.15-azs kube-proxy on Azure Stack

[jadarsie]

Reason for Change:

Something like this #2806 is required again to successfully deploy K8s 1.15 on air-gapped Azure Stack environments.

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3757 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3757   +/-   ##
=======================================
  Coverage   73.19%   73.20%           
=======================================
  Files         148      148           
  Lines       25367    25372    +5     
=======================================
+ Hits        18568    18573    +5     
  Misses       5663     5663           
  Partials     1136     1136           

Impacted Files	Coverage Δ
pkg/api/addons.go	97.80% <100.00%> (+0.01%)	⬆️

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[devigned]

/lgtm

pending passing tests

[acs-bot]

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