This change enables ephemeral encryption support to convert:
* encrypted source image to unencrypted destination image
* unencrypted source image to encrypted destination image
* encrypted source image to encrypted destination image
This also makes necessary changes for mypy checks to pass.
Related to blueprint ephemeral-storage-encryption
Change-Id: I9edc87006b1f7de69bc52f916f45c2cbb66abe23