Make instances optional since the API doesn't require this parameter.

[lonegunmanb]

Make instances optional since the API doesn't require this parameter.
If we leave this parameter nil in creation, the API will respond 0 when we read, so I add a default value to this argument.
This pr will fix #17821 .

The new added acc test:

=== RUN TestAccLinuxVirtualMachineScaleSet_defaultInstanceCount
=== PAUSE TestAccLinuxVirtualMachineScaleSet_defaultInstanceCount
=== CONT TestAccLinuxVirtualMachineScaleSet_defaultInstanceCount
--- PASS: TestAccLinuxVirtualMachineScaleSet_defaultInstanceCount (306.18s)
PASS

[stephybun]

Thanks for this @lonegunmanb, LGTM 🥮

[hui-dai]

Thank you for addressing this.

If "instances" is not set, it defaults to 0.

Will pluginsdk.ResourceData.HasChange("instances") return false in this case when checking the diff between "state" and "diff"?

[lonegunmanb]

Thank you for addressing this.

If "instances" is not set, it defaults to 0.

Will pluginsdk.ResourceData.HasChange("instances") return false in this case when checking the diff between "state" and "diff"?

Hi @hui-dai , no diff will be addressed in the plan, the acc test will apply a config file without setting instances, then execute plan to make sure that the config is idempotent.

[github-actions]

This functionality has been released in v3.17.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

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[hui-dai]

This PR does indeed make the instance parameter optional but it does not appear to have addressed the intent of the request in #17821, which is not to change the number of existing instances during an update action.

The semantics for not setting instances parameter need to be different for
resourceLinuxVirtualMachineScaleSetCreate
Intent: when instances is not set, default to 0
vs
resourceLinuxVirtualMachineScaleSetUpdate
intent: when instances is not set, default to null (Do not set the capacity for the SKU in the API call, see: https://docs.microsoft.com/en-us/rest/api/compute/virtual-machine-scale-sets/create-or-update?tabs=HTTP#sku)

By defaulting to 0, instead of null, this will resize VMSS down to 0 if the instance parameter is not specified.

[github-actions]

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