Permit cache-url to work with fifos.
Permit cache-url to work with fifos.

This makes it easier to work with temporary files - less traps
scattered around higher layer code.

Change-Id: I2fdd93115a7b0d289c2e28f8c133d4059de75b87

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