Fix cache-url to use single '=' in test expression
Fix cache-url to use single '=' in test expression

When downloading the base image for the first time, the response
test expression fails with '[: 200: unexpected operator'.  This is
caused by using '==' in the test expression, which is not supported
in sh scripts.

Fixes bug 1195030

Change-Id: I66260814cb591371dc5c10f8436f90c2f18d78cf

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