make disk gb required

Author: huangpf

src/ResourceManager/Compute/Commands.Compute/VirtualMachine/Config/AddAzureVMDataDiskCommand.cs

@@ -64,7 +64,7 @@ public class AddAzureVMDataDiskCommand : AzurePSCmdlet
         public string Caching { get; set; }
 
         [Parameter(
-            Mandatory = false,
+            Mandatory = true,
             Position = 4,
             ValueFromPipelineByPropertyName = true,
             HelpMessage = HelpMessages.VMDataDiskSizeInGB)]