refactor: enable generic --register-with-taints and --register-node

[jackfrancis]

Reason for Change:

This PR unwinds some weirdness in the implementation of --register-with-taints, especially for masterProfile configuration.

Also addressed some strange --register-node configuration.

Issue Fixed:

Requirements:

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

Notes:

[jackfrancis]

As far as I can tell this weird --register-node=true config inside this "any agent is Linux" block doesn't make sense. "true" is the default value for this, and we certainly never want to not register a node in the cluster.

Ref:

https://kubernetes.io/docs/reference/command-line-tools-reference/kubelet/

[codecov]

Codecov Report

Merging #3155 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3155      +/-   ##
==========================================
- Coverage   71.14%   71.13%   -0.01%     
==========================================
  Files         147      147              
  Lines       25664    25670       +6     
==========================================
+ Hits        18258    18261       +3     
- Misses       6269     6272       +3     
  Partials     1137     1137              

Impacted Files	Coverage Δ
pkg/api/common/const.go	40.00% <ø> (ø)
pkg/engine/templates_generated.go	38.05% <ø> (ø)
pkg/api/defaults-kubelet.go	96.42% <100.00%> (+0.13%)	⬆️
pkg/engine/template_generator.go	81.28% <0.00%> (-0.42%)	⬇️

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[mboersma]

/lgtm

(I think this is more of a refactor or even a feat than a chore.)