301-sql-alwayson-md-ilb-zones fails when ADE is enabled

[301-sql-alwayson-md-ilb-zones](https://github.com/Azure/azure-quickstart-templates/tree/master/301-sql-alwayson-md-ilb-zones)

### Issue Details
Modifying the deployment to work with Azure Disk Encryption causes the deployment to fail when creating the Storage Pool. This appears to be because the StartingDeviceId is hardcoded to 2, meaning that **xSqlCreateVirtualDataDisk NewVirtualDisk** in the DSC package PrepSQLAO.ps1 fails because it only returns a single drive, which isn't sufficient for a Storage Pool.

### Repro steps
1. Modify the VM templates to include Azure Disk Encryption
2. Deploy the DSC extension **prep-sqlao.ps1**
3. Deployment fails with the error message: _Unable to get 2 disks for creating Storage Pool_

Start with the 301-sql-alwayson-md-ilb-zones templates and the DSC package entry point prep-sqlao.ps1. Inspect xSqlCreateVirtualDataDisk NewVirtualDisk and its hardcoded StartingDeviceId, then reproduce the deployment with Azure Disk Encryption enabled. Done means the Storage Pool is created successfully when only one drive is returned.

Written by the indexing model from the issue text.