fix: aad-pod-identity taints foo works in k8s < 1.16

[jackfrancis]

Reason for Change:

This PR fixes two errata surfaced in 1.14 and 1.15 tests that include aad-pod-identity.

1. move the ensureTaints CSE func out of the "is k8s version >= 1.16?" block
2. use the documented kubectl taint nodes syntax (remove the =true expression after the taint key, which I guess works in 1.16+)

• Ref: https://kubernetes.io/docs/concepts/configuration/taint-and-toleration/

Issue Fixed:

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3199 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3199   +/-   ##
=======================================
  Coverage   71.23%   71.23%           
=======================================
  Files         147      147           
  Lines       25735    25735           
=======================================
  Hits        18332    18332           
  Misses       6258     6258           
  Partials     1145     1145           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	38.67% <ø> (ø)

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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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