DSC registration doesn't work when using generalized images

[201-vmss-automation-dsc](https://github.com/Azure/azure-quickstart-templates/tree/master/201-vmss-automation-dsc)
### Issue Details

I have a generalized/custom image I'm trying to provision the VMSS based upon it.
It works, but then only one node gets registered on the DSC Automation Account.
### Repro steps (_if necessary, delete otherwise_)
1. Deploy the template;
2. Verify that everything was deployed and registered under DSC Automation Account;
3. Delete the Resource Group created on the previous steps;
4. Change the current image in the template to a custom/generalized one;
5. Deploy the updated template again;
6. Verify that everything was deployed, but not all VMs were registered under DSC Automation Account.

Start with the linked 201-vmss-automation-dsc template and reproduce the deployment using a generalized/custom image after deleting the original resource group. Compare the VMSS instances with the nodes registered in the DSC Automation Account. Done means all provisioned VMs from the generalized image are registered under the DSC Automation Account.

Written by the indexing model from the issue text.