JoinDomain extension fails when using parameters/variables only

201-vm-domain-join
https://github.com/Azure/azure-quickstart-templates/tree/master/201-vm-domain-join
### Issue Details

I have not been able to get the JoinDomain extension to work when passing a parameter or variable value in for OUPath as in your example. The only thing I can get working is a concatenated string like this:
"OUPath": "concat('OU=Core,OU=xServers,DC=',parameters('clientPrefix'),',DC=domain,DC=com')]",

If I use "OUPath": "[variables('coreOuPath')]", the deployment fails even though I have the variable set exactly the same as above. I'm wondering if anyone else is experiencing this issue? I have tried various combinations with no success. The only way I get it to work is leaving the OUPath blank and creating the computer object ahead of time (not ideal) or have the machine go to the default Computers CN (also not ideal). I have the work around above but doesn't make for very efficient and re-usable templates. Let me know if I'm missing something.

Greg

Start with the 201-vm-domain-join template and compare the working concatenated OUPath expression with the failing variables('coreOuPath') and parameter-based forms. Confirm the expected behavior by deploying the template with a reusable parameter or variable value, with success meaning the VM joins the intended organizational unit without pre-creating the computer object.

Written by the indexing model from the issue text.