feat: add support for Kubernetes 1.19.0-rc.4

[mboersma]

Reason for Change:
See https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.19.md#changelog-since-v1190-rc3

Issue Fixed:

Requirements:

• Kubernetes artifacts built and pushed by Azure Pipelines
• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:
The Azure k8s artifacts are still building, so this will fail at first.

[codecov]

Codecov Report

Merging #3666 into master will decrease coverage by 0.00%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #3666      +/-   ##
==========================================
- Coverage   73.14%   73.14%   -0.01%     
==========================================
  Files         147      147              
  Lines       25165    25220      +55     
==========================================
+ Hits        18408    18448      +40     
- Misses       5623     5638      +15     
  Partials     1134     1134              

Impacted Files	Coverage Δ
pkg/api/common/const.go	40.00% <ø> (ø)
pkg/api/common/versions.go	96.84% <ø> (ø)
pkg/engine/engine.go	85.64% <0.00%> (-0.31%)	⬇️
pkg/engine/templates_generated.go	53.26% <0.00%> (-0.36%)	⬇️
pkg/api/vlabs/validate.go	79.66% <87.50%> (+0.16%)	⬆️
pkg/api/addons.go	97.79% <100.00%> (+0.02%)	⬆️
pkg/api/k8s_versions.go	100.00% <100.00%> (ø)
pkg/engine/artifacts.go	99.14% <100.00%> (+0.01%)	⬆️

---

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[mboersma]

Still working out a kind-related issue with the build pipeline, and then we will re-run CI once artifacts are published.

[mboersma]

It looks like the AKS-Engine WS2019 VHD - docker job is taking just over two hours in this case (and 1:57 on some other PRs where it passed). So I added a commit that gives that job 2.5 hours. 🤞

[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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