Add field for group id in user add dialog
    
    Add new field in user add dialog. This combo box lists all posix groups
    so user can choose one. It is also possible to fill a GID number
    which is not in the list.
    
    https://fedorahosted.org/freeipa/ticket/5505
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>