[web2py] instruct web2py only to alter table.

[vortex]

What should be the definition so that web2py only alters the mysql
table to correspond to the new definition but if it exists not to
create it.

            db_server.define_table('db_version',
                Field('major', 'integer', default = 0),
                Field('minor', 'integer', default = 0),
                migrate = True,
            )

Or at least never to create the table but alter it if needed.