Allow absolute path to image with ironic-agent
Allow absolute path to image with ironic-agent

The element was prepending the work directory forcibly, which will have
problems when the image name has an absolute path, which is allowed.

Change-Id: I0cb7b96e24daab8ee73611936af72074c70ac1aa
Closes-Bug: #1400405

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