Don't verify image hashes if checksumming is disabled.
Don't verify image hashes if checksumming is disabled.

There was a bug where we were checking base image checksums, even
if checksumming was disabled, if there was already a cached checksum
for the image. This resolves bug 1075017.

DocImpact

Change-Id: I38f5914cd9d6326fdf5e5d7f0f57885ff8b5ff32

n0ano/ganttclient