Barbican secret deletion support

The Block Storage Service (Cinder) offers end users the ability to create images from encrypted volume types. When it does this, Cinder stores a secret in Barbican in a 1-1 relation to the image and puts the Barbican ID of the secret as an image property,
 `cinder_encryption_key_id``. When a user deletes such an image, the Barbican secret is no longer applicable to any resource, but it persists in Barbican. As a result, deployers have a situation where useless secrets are piling up in Barbican. This would be mitigated if Glance deleted the unique Barbican secret of an image at the time of image deletion.