Update default RHEL guest image
Update default RHEL guest image

Red Hat periodically updates the qcow2 guest image available
for download. This sets the default image name to the latest.

Change-Id: Iba3075bbee3b41918d5cd3da9721fcbf98ff3bcd

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