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>