Fixes#7053 where, when using `additional_unattend_config` in
`os_profile_windows_config` we got an error as follows:
```
azurerm_virtual_machine.test: [DEBUG] Error setting Virtual Machine
Storage OS Profile Windows Configuration: &errors.errorString{s:"Invalid
address to set: []string{\"os_profile_windows_config\", \"1534614206\",
\"additional_unattend_config\"}"}
```
adminPassword
The Azure API never returns the AdminPAssword (as is correct) from the
Read API call. Therefore on Create, we do not set the AdminPassword of
the vm as part of the state. The Same func is used for Create & Update,
therefore when we changed anything on the VM, we were getting the
following error:
```
statusCode:Conflict
serviceRequestId:f498a6c8-6e7a-420f-9788-400f18078921
statusMessage:{"error":{"code":"PropertyChangeNotAllowed","target":"adminPassword","message":"Changing property 'adminPassword' is not allowed."}}
```
To fix this, we need to excldue the AdminPassword from the Update func
if it is empty
Paul Hinze