Comment 2 for bug 1942682

This results in a problem in cinder, which asserts the exact value to determine whether the image is encrypted or not.
  https://github.com/openstack/cinder/blob/fff0ee68a8b1b5210038a272e9751405250b6b01/cinder/volume/flows/manager/create_volume.py#L562

Honestly speaking the current format(true/false) looks more proper and we should fix how cinder handles the attribute but changing the value format would not be ideal.