Fix the DIB_OFFLINE setting to actually work.

I missed the getopt parameter and forgot defaults are imported after
option processing. Untested code is broken code!

Change-Id: I133a691909d38e834c204950276a57f4884fc4ed
Enable --offline support for Ubuntu root images.

When --offline is set elements should not revalidate cached data. The
ubuntu element had not been updated to match this. SHA checking is
also skipped as we only move a new cached file into place when the
hash matches, and we might download a new hash before updating the
image cache, which would cause persistent --offline failures.

Change-Id: If1a0366b51951a73b7a3ffe23a29a3d910b08938
Update the Fedora element to honour --offline.

Similar to the Ubuntu element the Fedora element had not been updated
to honour --offline. Also similarly we only check for the existence of
the final cached file rather than the image we directly download,
because they are generated separately.

The -u change is just hygiene to detect future buggy changes.

Change-Id: Ib92a9d419bb485c8861da041066827e8d6aac0b7