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>
In FlaskSQLAlchemy, `db.Table` is not a class, but a function mimicking the Table interface, hence the definition above.
https://pythonhosted.org/Flask-SQLAlchemy/api.html ("Check types carefully")

We will add comments though.