Update the Fedora element to honour --offline.
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

openstack/diskimage-builder,zioproto/diskimage-builder,citrix-openstack-build/diskimage-builder,rdo-management/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,JioCloud/diskimage-builder,eantyshev/diskimage-builder,rdo-management/diskimage-builder,eantyshev/diskimage-builder,citrix-openstack-build/diskimage-builder,openstack/diskimage-builder,varunarya10/diskimage-builder,zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,varunarya10/diskimage-builder,takahashinobuyuki/diskimage-builder,JioCloud/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder,NaohiroTamura/diskimage-builder