fix: add retry when downloading files in windows CSE

[marosset]

Reason for Change:

Replicating #4206 for the Windows CSE

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

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

Notes:

[jsturtevant]

there are a few other places as well: https://github.com/Azure/aks-engine/search?q=Invoke-webrequest

Might as well get them all 😄

[codecov]

Codecov Report

Merging #4232 (c7398ed) into master (f5836e0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4232   +/-   ##
=======================================
  Coverage   72.07%   72.07%           
=======================================
  Files         141      141           
  Lines       21698    21698           
=======================================
  Hits        15638    15638           
  Misses       5103     5103           
  Partials      957      957           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	43.42% <ø> (ø)

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[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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