Make dependsOn check more granular to improve parallelism and reduce cascading failures

The nested template files under /ARM-wvd-templates/nestedtemplates/ , such as
[ARM-wvd-templates/nestedtemplates/managedDisks-galleryvm.json](https://github.com/Azure/RDS-Templates/blob/master/ARM-wvd-templates/nestedtemplates/managedDisks-galleryvm.json) should update the joindomain and dscextension resources declarations to depend on only on the vm it needs to run on instead of all VMs. The advantage of this is that the join domain and dscextension can run on an individual vm as soon as it is ready, also if one vm fails at some point it won't cause all of the other vms to fail like it does now.

For example for joindomain instead of
`"dependsOn": ["rdsh-vm-loop"],`
it would be something like
```
"dependsOn": [
"[concat('Microsoft.Compute/virtualMachines/', variables('rdshPrefix'), copyindex(), parameters('rdshPrefix'), add(copyindex(), parameters('vmInitialNumber')) )]"
],
```

This change is already implemented here [wvd-templates/Create and provision WVD host pool/mainTemplate.json](https://github.com/Azure/RDS-Templates/blob/wvd-templates_preview_20200716.1800.0/wvd-templates/Create%20and%20provision%20WVD%20host%20pool/mainTemplate.json) , although ignore the special case for the 0 index as that doesn't apply to this template.

FYI @nakranimohit0

Start with ARM-wvd-templates/nestedtemplates/managedDisks-galleryvm.json and inspect the joindomain and dscextension resource declarations. Compare their dependsOn expressions with the implementation in wvd-templates/Create and provision WVD host pool/mainTemplate.json, excluding its special case for index 0. Done means the declarations depend on only the VM they run on across the applicable nested templates.

Written by the indexing model from the issue text.