Compute codegen fails with default tag package-2018-06-exclude-gallery

The tag [package-2018-06-exclude-gallery](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/compute/resource-manager/readme.md#tag-package-2018-06-exclude-gallery) contains following two swaggers:

Microsoft.Compute/stable/2018-06-01/compute.json
Microsoft.Compute/stable/2018-04-01/disk.json

These two swaggers defines same enum type with different values which is in-correct.

[StorageAccountTypes](https://github.com/Azure/azure-rest-api-specs/blob/38cdfe02f462176e369540a7dcd504bad3137ce4/specification/compute/resource-manager/Microsoft.Compute/stable/2018-06-01/compute.json#L5175) in compute.json
[StorageAccountTypes](https://github.com/Azure/azure-rest-api-specs/blob/38cdfe02f462176e369540a7dcd504bad3137ce4/specification/compute/resource-manager/Microsoft.Compute/stable/2018-04-01/disk.json#L790) in disks.json

This needs to be fixed in order to be able to generate the code.

Renaming the storage account type used in disks.json to DiskStorageAccountTypes is one way to fix this (since we did similar [rename](https://github.com/Azure/azure-rest-api-specs/blob/38cdfe02f462176e369540a7dcd504bad3137ce4/specification/compute/resource-manager/Microsoft.Compute/stable/2018-06-01/disk.json#L791) in latest version of disks.json)

Start with the package-2018-06-exclude-gallery tag in specification/compute/resource-manager/readme.md, then compare StorageAccountTypes in Microsoft.Compute/stable/2018-06-01/compute.json and Microsoft.Compute/stable/2018-04-01/disk.json. Verify the enum names and values no longer conflict, and confirm code generation succeeds for the default tag.

Written by the indexing model from the issue text.