Last ditch effort to correct a wrong shasum.

Some users on corp networks are finding that cloud images appear in
their proxies before SHA256SUMS files.
We now try to force any to upstream proxies to update their copy and
retry the checksum, rather than immediately failing.
We also now add proper argument parsing to cache-url, and associated
help functionality, since we have now grown our first argument.

Change-Id: I44d44b62db8d1df990606fdf087ec1b837f491f6
Closes-Bug: #1336067
Merge "Last ditch effort to correct a wrong shasum."