Enhance dib-run-parts usage message
Enhance dib-run-parts usage message

and fix $1: unbound variable issue when run dib-run-parts --list.

Change-Id: Id18e21683f785f318bce11d401cb4f826bed1b01

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