refactor: consolidate flannel addon spec

[jackfrancis]

Reason for Change:

Eases maintenance of the flannel addon by consolidating manifests into a single YAML file that uses Go templating.

Issue Fixed:

Addresses #3188.

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3385 into master will increase coverage by 0.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3385      +/-   ##
==========================================
+ Coverage   72.60%   72.70%   +0.10%     
==========================================
  Files         147      147              
  Lines       25206    25170      -36     
==========================================
  Hits        18300    18300              
+ Misses       5773     5737      -36     
  Partials     1133     1133              

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	49.16% <0.00%> (+1.06%)	⬆️

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[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma

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