refactor: remove unsupported orchestrators

[mboersma]

Reason for Change:

Removes references to the Swarm, Swarm mode, Mesos, and DC/OS orchestrator types to simplify the AKS Engine codebase.

This code was already effectively dead, but was being retained out of an abundance of caution in order not to break AKS' vendoring needs. That is no longer a concern.

Issue Fixed:

Refs #2500

Credit Where Due:

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

• No
• Yes

Requirements:

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

Notes:

Let me know if I should make separate PRs for each commit instead to make it more digestible.

There is further refactoring for simplicity that can be done here, probably starting with removal of GenerateTemplate and any remaining switch statements where the only case is Kubernetes now. This seems like enough to review, however.

[jackfrancis]

cc @xuto2

[codecov]

Codecov Report

Merging #3965 (248d010) into master (3fa95be) will decrease coverage by 0.65%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #3965      +/-   ##
==========================================
- Coverage   73.72%   73.06%   -0.66%     
==========================================
  Files         147      145       -2     
  Lines       23177    21856    -1321     
==========================================
- Hits        17087    15969    -1118     
+ Misses       4976     4849     -127     
+ Partials     1114     1038      -76     

Impacted Files	Coverage Δ
pkg/api/common/const.go	100.00% <ø> (+60.00%)	⬆️
pkg/api/common/versions.go	97.38% <ø> (+1.00%)	⬆️
pkg/api/converterfromapi.go	95.65% <ø> (+0.78%)	⬆️
pkg/api/convertertoapi.go	93.49% <ø> (-1.11%)	⬇️
pkg/api/defaults-custom-cloud-profile.go	85.24% <ø> (-0.47%)	⬇️
pkg/api/mocks.go	100.00% <ø> (ø)
pkg/api/types.go	92.82% <ø> (-0.89%)	⬇️
pkg/api/vlabs/types.go	72.53% <ø> (-1.30%)	⬇️
pkg/engine/params.go	63.63% <ø> (-2.83%)	⬇️
pkg/engine/template_generator.go	67.59% <0.00%> (-13.88%)	⬇️
... and 12 more

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[mboersma]

I think we might be exceeding the custom data size limit with the 1_20_docker_e2e config: CI fails there consistently in this PR.

[jackfrancis]

/lgtm

🎉

[acs-bot]

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