If an image is encrypted, we will need to retrieve the passphrase from
the key manager service in order to create an instance from it.
This adds an image property to store the secret UUID that belongs to
the image. It will only be used to decrypt the image and will not be
used to encrypt or decrypt any other image. Nova will create a new
secret for each disk image it creates, including snapshots.
Related to blueprint ephemeral-storage-encryption
Change-Id: I01eef6adc2c8feb64e86b33392b8b4b483041e27