This change fixes an issue where using the `disk_additional_size` configuration option would cause builds to fail.
Build results before the change
```
==> Builds finished but no artifacts were created.
Build 'azure-arm' errored: Code="DeploymentFailed" Message="At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/DeployOperations for usage details." Details=[{"code":"BadRequest","message":"{\r\n \"error\": {\r\n \"code
\": \"InvalidParameter\",\r\n \"message\": \"The entity name 'dataDisk.name' is invalid according to its validation rule: ^[^_\\\\W][\\\\w-._]{0,79}(?\u003c![-.])$.\",\r\n \"target\": \"dataDisk.name\"\r\n }\r\n}"}]
```
Build results after change
```
Build 'azure-arm' finished.
==> Builds finished. The artifacts of successful builds are:
--> azure-arm: Azure.ResourceManagement.VMImage:
OSType: Linux
ManagedImageResourceGroupName: test-pkr
ManagedImageName: wilkenPacker9249
```
Closes#9249
