MachinePool always update vmssState

[mweibel]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Updates ScaleSets service to always update the scope's vmss state with the latest data.
Previously, a long running operation would cause the vmssState to not be updated and delayed creation of AzureMachinePoolMachines until much later when that long running operation completed.
This change avoids this and updates the vmssState to what is retrieved from the API at all times.
The call does not make more API requests because the VMSS GET request was done anyway but its result ignored.

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 kubernetes-sigs/cluster-api#9858 for CAPZ

Special notes for your reviewer:

Question: Should I add a test which tests this specific case with a long running operation? Is there any existing test which resembles this?

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Improves state update handling when running long operations on VMSS

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @mweibel. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[mweibel]

FYI this is a small refactoring I noticed while looking through the tests - ReconcileReplicas never returns an error and I adjusted its signature therefore.

[willie-yao]

/ok-to-test