[release-1.15] handle non-native autoscalers for AzureASOManagedMachinePool

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #4929

/assign nojnhuh

Fixed a bug causing ASO and non-AKS-native autoscalers to fight over the replica count of an AzureASOManagedMachinePool

[mboersma]

/lgtm /approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: f3239106e156c0cc51f5ca9c684ea08bb57e0b3f

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cluster-api-provider-azure/blob/release-1.15/OWNERS)~~ [mboersma] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 62.00%. Comparing base (805be18) to head (4918a39). Report is 1 commits behind head on release-1.15.

Files	Patch %	Lines
exp/mutators/azureasomanagedmachinepool.go	75.00%	1 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.15 #4935 +/- ## ================================================ - Coverage 62.01% 62.00% -0.02% ================================================ Files 201 201 Lines 16861 16867 +6 ================================================ + Hits 10456 10458 +2 - Misses 5622 5625 +3 - Partials 783 784 +1 ```

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[nojnhuh]

Bumping codecov seemed to work to get that passing. I'll revert that change here and open a separate PR.

[nojnhuh]

I'll revert that change here and open a separate PR.

done and done: #4938

[nojnhuh]

Rebased this to include #4938.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3c7d4c40577e495665dc7097830fe4dca2d275e9

[nojnhuh]

/kind bug