feat: add the aksEngineVersion tag

[mathieu-benoit]

Reason for Change:

VMSS agent VMs are missing the "aksEngineVersion" : "[parameters('aksEngineVersion')]", tag that VMAS VMs have (https://github.com/Azure/aks-engine/blob/master/parts/k8s/kubernetesmasterresourcesvmss.t#L415-L416).

Issue Fixed:

Fixes #411

Requirements:

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

Notes:

[tariq1890]

@CecileRobertMichon @jackfrancis Thoughts on this PR? I am mostly okay with it, but don't have strong opinions given that we already tag the master VMs.

[tariq1890]

Deferring to others for final approval.

[CecileRobertMichon]

@tariq1890 see #411 for context. Mathieu noticed the tag was missing on AKS VMSS preview VMs.

Thanks @mathieu-benoit for opening the PR!

/lgtm

[CecileRobertMichon]

updated description

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[codecov]

Codecov Report

Merging #413 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #413   +/-   ##
=======================================
  Coverage   53.42%   53.42%           
=======================================
  Files          95       95           
  Lines       14363    14363           
=======================================
  Hits         7673     7673           
  Misses       6027     6027           
  Partials      663      663

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