[release-1.10] fix endless reconciliation on AKS system node labels

[nojnhuh]

What type of PR is this?
/kind bug
/area managedclusters

What this PR does / why we need it: Manual cherry-pick of #3975.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Fixed a bug where AzureManagedMachinePools using Spot instances would endlessly reconcile without any user-specified node labels

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bc99dde) 54.27% compared to head (a8ffe6e) 54.27%.
Report is 1 commits behind head on release-1.10.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-1.10    #3989   +/-   ##
=============================================
  Coverage         54.27%   54.27%           
=============================================
  Files               186      186           
  Lines             18861    18866    +5     
=============================================
+ Hits              10237    10240    +3     
- Misses             8069     8071    +2     
  Partials            555      555           

Files Changed	Coverage Δ
azure/services/agentpools/spec.go	58.91% <100.00%> (+1.04%)	⬆️

... and 1 file with indirect coverage changes

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[mboersma]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 69da9ee38c7c0a8671eba3562b025fbf37af8084

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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