Usage message enhancement
Usage message enhancement

Add --help, add a simple usage and two examples

Change-Id: Iddf07ec1935f337000bd8ffb3d7c1fcc65753e8d

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