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>
I would use directly http://docs.sqlalchemy.org/en/rel_0_9/core/metadata.html#sqlalchemy.schema.Table to make sure that we will not conflict with https://github.com/mitsuhiko/flask-sqlalchemy/blob/6fd538e27a9eb65cfb06d36632833cc6297d7810/flask_sqlalchemy/__init__.py#L60-L68