Create an new AD Domain with 2 Domain Controllers fails validation

Create an new AD Domain with 2 Domain Controllers
https://github.com/Azure/azure-quickstart-templates/tree/master/active-directory-new-domain-ha-2-dc/
by Simon Davies
Last updated: 7/5/2018

### Issue Details
After loading this template in Visual Studio 2018, validation fails for CreateADPDC.pst.

CreateADPDC.ps1:67 char:13
16:29:56 - + DiskNumber = 2
16:29:56 - + ~~~~~~~~~~
16:29:56 - The member 'DiskNumber' is not valid. Valid members are
16:29:56 - 'AllocationUnitSize', 'AllowDestructive', 'ClearDisk', 'DependsOn', 'DiskId', 'DiskIdType', 'DriveLetter', 'FSFormat',
16:29:56 - 'FSLabel', 'PsDscRunAsCredential', 'Size'.

### Repro steps (*if necessary, delete otherwise*)
1. In Visual Studio create a new project(Azure Resource Group)
2. Select "active-directory-new-domain-ha-2-dc" By: simongdavies
3. Validate

Open active-directory-new-domain-ha-2-dc/CreateADPDC.ps1 and inspect line 67, where validation reports the invalid DiskNumber member. Reproduce the validation failure in Visual Studio 2018 using the listed template, then verify that validation succeeds after the relevant compatibility issue is addressed.

Written by the indexing model from the issue text.