Always export IMAGE_NAME
Always export IMAGE_NAME

A recent change[1] relies on IMAGE_NAME being available when
98-source-repositories was run. However IMAGE_NAME was previously only
exported if the -o option had been used.

[1] I7dbe9e163ad38a418cf2869a81e720de2c27dfb1

Change-Id: Ife3d94fd45a2e0e0948c17414c369e0e6e040442

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