[azure-arm] Allow specifying azure availability zone in the packer template

_This issue was originally opened by @miketimofeev as hashicorp/packer#10648. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

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#### Description
Recently we've started experiencing issues during VM creation in Azure without specifying availability zone:
`azure-arm: resources.DeploymentsClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="InvalidTemplateDeployment" Message="The template deployment failed with error: 'The resource with id: '/subscriptions/***/resourceGroups/99275_ubuntu1604/providers/Microsoft.Compute/virtualMachines/pkrvmkeubkm2jhf' failed validation with message: 'The requested size for resource '/subscriptions/***/resourceGroups/99275_ubuntu1604/providers/Microsoft.Compute/virtualMachines/pkrvmkeubkm2jhf' is currently not available in location '***' zones '' for subscription '***'. Please try another size or deploy to a different location or zones. See https://aka.ms/azureskunotavailable for details.'.'."`

The same happens when trying to create VM through the Azure portal with the following settings:

However, VM creates just fine using any available zone:

In the azure deploying template it looks like this:
```
"zones": [
"[parameters('zone')]"
]

```

Unfortunately, we can't find such parameter in packer templates so our VM generation is blocked and we're trying to solve the issue through Azure support.

#### Use Case(s)

See above

#### Potential configuration

```
"type": "azure-arm",
"location": "westus2",
"az": "1",
```

#### Potential References

No source files or tests are named. Start by locating the azure-arm template handling and Azure deployment generation, then compare the proposed "az" setting with the deployment's zones parameter. Done means a configured availability zone is accepted and used during VM creation.

Written by the indexing model from the issue text.