Multi attribute with comma cause unexpected behavior

Hi,

When an object have attributes declared many time each one is append to a list. For example

```
service {
  ...
  notification_options c,r,f
  notification_options c,r,w,f
  ...
}
```

Service have as notification_options attribute ```[u'c,r,f', u'c,r,w,f']``` and of course when we test

```
if 'c' in service.notification_options:
```

we will never have notification.

In order to solve this bug i've patch the ```to_split``` function in order to always return list which doesn't contain other list.

I don't know if it's the best solution but actually with default template no service notification can go due to previous example. 

Thank you for reading :)