If checksum and size provided in image meta data, fuel-agent will do
checksum comparing to ensure that data written on disk and downloaded
image are identical.
Partially implements blueprint: ibp-image-checksums
Depends on: Ia9c8325a2f71af2a1af2ad54a1ae414edf49a7c6
Change-Id: Id120848537cfc4ed908b9e524624855fb7308603