readonly flag in volume_admin_metadata results in ro mode attachments every time

[Walt Boring]

Regardless of the value of the 'readonly' property in the volume_admin_metadata table for a volume, this results in the attachment being created as 'ro'. This is due to the value in the database table being kept as a string, and an if comparison of a string always results in True if the string has any characters.

This is the source of the issue and the string comparison to True:

https://github.com/openstack/cinder/blob/master/cinder/volume/api.py#L2461-L2469

```
        # Use of admin_metadata for RO settings is deprecated
        # switch to using mode argument to attachment-create
        if self.db.volume_admin_metadata_get(
                ctxt.elevated(),
                volume_ref['id']).get('readonly', False):
            LOG.warning("Using volume_admin_metadata to set "
                        "Read Only mode is deprecated! Please "
                        "use the mode argument in attachment-create.")
            attachment_ref.attach_mode = 'ro'
            # for now we have to let the admin_metadata override
            # so we're using an else in the next step here, in
            # other words, using volume_admin_metadata and mode params
            # are NOT compatible
```

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/905317