[release-1.9] Don't default NAT Gateway for existing node subnet

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #3700

/assign CecileRobertMichon

Don't default NAT Gateway for existing node subnet

[CecileRobertMichon]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 2299b73157b57f79c9816fc4c91ab60470be6d2e

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon

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[codecov]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.07 🎉

Comparison is base (a6bda5c) 52.83% compared to head (11adba6) 52.90%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.9    #3711      +/-   ##
===============================================
+ Coverage        52.83%   52.90%   +0.07%     
===============================================
  Files              182      182              
  Lines            18245    18250       +5     
===============================================
+ Hits              9639     9655      +16     
+ Misses            8067     8055      -12     
- Partials           539      540       +1     

Impacted Files	Coverage Δ
azure/services/subnets/spec.go	80.26% <82.60%> (+17.94%)	⬆️
api/v1beta1/azurecluster_default.go	91.20% <100.00%> (-0.06%)	⬇️

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[CecileRobertMichon]

/retest