sqlalchemy: default mysql parameters for db.Table

* db.Table is InnoDB by default on modern versions
  of MySQL. Hence, foreign keys could not be set
  on db.Model fields because the latter are
  hardcoded to MyISAM. (closes #2491)

Signed-off-by: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Reviewed-by: Gilles Louppe <gilles.louppe@cern.ch>
Could you add more comments here? Or create the `Table` subclass to make it more obvious?

```python

class Table(original_table):

    """Override MySQL parameters to force MyISAM engine."""

    def __init__(self, *args, **kwargs):
        """Use same MySQL parameters that are used for ORM models."""
        kwargs.update(mysql_parameters)
        return super(Table, self).__init__(*args, **kwargs)
```