test: UT for addons ip-masq-agent image override

[jackfrancis]

Reason for Change:

This PR adds a unit test to prove that when you upgrade to 1.16, your custom ip-masq-agent addon image value will be overridden to use the aks-engine default.

Issue Fixed:

Requirements:

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

Notes:

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

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[codecov]

Codecov Report

Merging #2997 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2997   +/-   ##
=======================================
  Coverage   72.12%   72.12%           
=======================================
  Files         141      141           
  Lines       26257    26257           
=======================================
  Hits        18937    18937           
  Misses       6227     6227           
  Partials     1093     1093           

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