[FIX] let add_fields() work for Odoo <= v8
[FIX] let add_fields() work for Odoo <= v8

In those versions, [there is a required column called `select_level`][1] whose [default value is `"0"`][2].

I'm adding it to `add_fields()` so it doesn't crash when trying to use it in those versions.

The crash looks like this:

```
2020-03-12 03:48:38,493 1 ERROR prod openerp.sql_db: bad query:
            INSERT INTO ir_model_fields (
                model_id, model, name, field_description, ttype, state
            ) VALUES (
                117, 'product.template', 'pack', 'OU', 'bool', 'base'
            ) RETURNING id
Traceback (most recent call last):
  File "/opt/odoo/custom/src/odoo/openerp/sql_db.py", line 247, in execute
    res = self._obj.execute(query, params)
IntegrityError: null value in column "select_level" violates not-null constraint
DETAIL:  Failing row contains (9885, product.template, 117, pack, null, null, OU, bool, base, null, f, null, f, null, null, null, null, null, null, null, null, null, null, null, null, null).

2020-03-12 03:48:38,493 1 ERROR prod OpenUpgrade: Error after 0:00:00.000372 running
            INSERT INTO ir_model_fields (
                model_id, model, name, field_description, ttype, state
            ) VALUES (
                117, 'product.template', 'pack', 'OU', 'bool', 'base'
            ) RETURNING id
2020-03-12 03:48:38,493 1 ERROR prod OpenUpgrade: product_pack: error in migration script product_pack/migrations/8.0.1.0.0/pre-migration.py: IntegrityError('null value in column "select_level" violates not-null constraint\nDETAIL:  Failing row contains (9885, product.template, 117, pack, null, null, OU, bool, base, null, f, null, f, null, null, null, null, null, null, null, null, null, null, null, null, null).\n',)
2020-03-12 03:48:38,493 1 ERROR prod OpenUpgrade: null value in column "select_level" violates not-null constraint
```

[1]: https://github.com/odoo/odoo/blob/9e8f70e4849b0eeaca8b5cf51372ecfa23dc561b/openerp/addons/base/ir/ir_model.py#L261
[2]: https://github.com/odoo/odoo/blob/9e8f70e4849b0eeaca8b5cf51372ecfa23dc561b/openerp/addons/base/ir/ir_model.py#L286

@Tecnativa TT22023 TT18838

OCA/openupgradelib