[schema] store default attribute values in a Bytes field, allowing python objects as default values Notes: * Binary objects grow two methods * we depend on a newer yams version * the code is quite simplified as a result of the whole change * a very old and nasty bug where bool(default values) evaluating to False not being properly migrated is killed Closes #2414591.

if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',
           ask_confirm=False):
    rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',
        ask_confirm=False)