chore: deprecate kube-dashboard addon

[jackfrancis]

Reason for Change:

This PR adds a deprecation warning whenever the kube-dashboard addon is enabled, and disables testing of this going forward.

The justification for this is that the Kubernetes version support matrix for dashboard releases is somewhat complicated, and would require an expanded aks-engine test matrix in order to reliably produce a known-working implementation across the set of supported Kubernetes versions that aks-engine can create clusters with. See:

https://github.com/kubernetes/dashboard/releases

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:

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

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• OWNERS [jackfrancis]

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[jackfrancis]

cc @mboersma @jadarsie @craiglpeters

[codecov]

Codecov Report

Merging #4268 (a08751a) into master (6726888) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4268   +/-   ##
=======================================
  Coverage   72.02%   72.03%           
=======================================
  Files         141      141           
  Lines       21625    21627    +2     
=======================================
+ Hits        15576    15578    +2     
  Misses       5099     5099           
  Partials      950      950           

Impacted Files	Coverage Δ
pkg/api/common/const.go	100.00% <ø> (ø)
pkg/api/k8s_versions.go	100.00% <ø> (ø)
pkg/engine/templates_generated.go	43.56% <ø> (ø)
pkg/api/vlabs/validate.go	81.46% <100.00%> (+0.03%)	⬆️

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[mboersma]

This lgtm generally, but it also deletes

• docs/topics/kubernetes-walkthrough.md
• docs/topics/monitoring.md

which doesn't seem like a related change. Could those go in a separate PR?

[jackfrancis]

@mboersma good idea:

#4287

(That removal was identified as a part of this particular PR spike because it recommends installing the dashboard.)