When first generating user groups xml, user SUPERUSER's context

Presently, on non-english systems, groups on the user's form have English names.

This patch allows translation at install time according to the superuser's context at install time

As of 8.0 the context is frozen in many cases, so as a good practice please clone it rather than altering it in place (less work when porting to 8.0). The following should be sufficient, right?
```python
    context = dict(context or {}, self.pool['res.users'].context_get(cr, uid))
```