[release-1.10] fix vnet in separate resource group

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #4191

/assign nojnhuh

Fixed a bug where AzureCluster's spec.networkSpec.vnet.resourceGroup was not always properly honored

[CecileRobertMichon]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 0fc1426faa733af02332504797deaba3f5b27bfa

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cebffa2) 54.37% compared to head (0d32599) 54.37%.
Report is 1 commits behind head on release-1.10.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-1.10    #4207   +/-   ##
=============================================
  Coverage         54.37%   54.37%           
=============================================
  Files               186      186           
  Lines             18896    18896           
=============================================
  Hits              10274    10274           
  Misses             8067     8067           
  Partials            555      555           

Files	Coverage Δ
azure/scope/cluster.go	56.81% <100.00%> (ø)

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