WaitForDisk: Add read-only parameter 'Available'

[johlju]

Details of the scenario you tried and the problem that is occurring:
Suggest we add a read-only parameter to the WaitForDisk resource. That way the Get-TargetResource could call Test-TargetResource and return the result (the actual current state) of the disk in the property Available when calling Get-DscConfiguration.

Alternative names for the read-only property could be 'IsAvailable' or 'IsPresent'.

The DSC configuration that is using the resource (as detailed as possible):
n/a

Version of the Operating System and PowerShell the DSC Target Node is running:
n/a

Version of the DSC module you're using, or 'dev' if you're using current dev branch:
Dev