Really handle Ubuntu mirror cache skew,

The last patch to do this only handled the case of SHA256SUM being
stale, but the image is more likely to be stale in the default
configuration as the hash file is downloaded over HTTPS.

Change-Id: I6531fa684e560cad48c3696394d48a600680c875
Closes-Bug: #1336067
Merge "Really handle Ubuntu mirror cache skew,"