groups: port missing functionality

* Implement user group manage function

* Add filter on list of group that user can join.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
I can add this:
```python
Usergroup.admins = db.relationship(
    UserUsergroup,
    primaryjoin="and_(Usergroup.id==UserUsergroup.id_usergroup, " +
                "UserUsergroup.user_status=='%s', UserUsergroup.id_user)"
    % UserUsergroup.USER_STATUS['ADMIN'])
```
But I do not know if we can have great advantages compared to having a method `get_admins()`.