Prevent creating encrypted volume with image

Prevent creating encrypted volume with image

When creating an encrypted volume with a unencrypted image,
cinder just copies the unencrypted data to the volume,
which keeps unencrypted data in the volume.
But when booting/attaching the volume to Nova, it thinks
encrypted data on the volume. As a result, Nova reads
the data incorrectly.

The fix is to prevent such action. Later, we can
implement to write encrypted data in encrypted volumes
when creating from unencrypted image.

Change-Id: I0af8318222cee023451c38812bf83cdb4086faae
Closes-bug: #1482464