Please fix validateset for pd-balanced on type param for New-GCEDisk function

[mazad01]

Hello,

When trying to pass pd-balanced as a disktype to the New-GCEDisk function, I get the following:

New-GceDisk : Cannot validate argument on parameter 'DiskType'. The argument "pd-balanced" does not belong to the set "pd-ssd,pd-standard" specified by the ValidateSet attribute. Supply an argument that is in the set and then try the command again.

Looks like it will need to be added here: https://github.com/GoogleCloudPlatform/google-cloud-powershell/blob/master/Google.PowerShell/Compute/GceDisk.cs#L221