[release-1.15] Set cluster name label for pre-existing kubeconfig

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #4833

/assign willie-yao

Set cluster name label for pre-existing kubeconfig

[mboersma]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 55ac12c49a048c81f5ee5930d252c80c3eab492c

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 62.00%. Comparing base (e1f1f75) to head (9fc6a52).

Files	Patch %	Lines
controllers/azuremanagedcontrolplane_reconciler.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.15    #4848      +/-   ##
================================================
- Coverage         62.02%   62.00%   -0.03%     
================================================
  Files               201      201              
  Lines             16857    16861       +4     
================================================
- Hits              10456    10454       -2     
- Misses             5618     5624       +6     
  Partials            783      783              

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