fixes #7331 - delete unassigned os default templates

**The problem**: Previously assigned config template is not deleted when unassigning it. Therefore, can never delete the config template (the error will raise that it belongs to os_default_template).
**The fix**: adds ```ruby :_destroy => 1``` to existing os_default_template, if the config_template_id is blank.
Maybe you want to substitute this by:

```
assert_difference('@os.os_default_templates.length', -1) do
   @os.update_attributes(:os_default_templates_attributes => { :id => os_default_template.id, :_destroy => 1 })
end
```

and avoid ```initial_os_default_templates_length```, it's a little easier to read in my opinion.