Fix tftp mapping on openSUSE
Fix tftp mapping on openSUSE

the atftp package is not compatible with what TripleO expects

Change-Id: I3309de2bb445bba1ab5ed000e87583387b519f2d

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