Use conditional GET to fetch latest ubuntu image.
Use conditional GET to fetch latest ubuntu image.

This also switches to using curl which some people may not have
installed. However, curl is far superior for this type of download.

Change-Id: I7ac5a84b30eb8daad320c082f976931c41a24669

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