sqlalchemy: default mysql parameters for db.Table

* Adds default parameters to `db.Table` constructor to force MyISAM
  engine for MySQL backend. db.Table is InnoDB by default on modern
  versions of MySQL. Hence, foreign keys can now be set on
  `db.Model` fields because both are using MyISAM as an engine.
  (closes #2491)

Signed-off-by: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Reviewed-by: Gilles Louppe <gilles.louppe@cern.ch>