debian: properly deal with Debian stable/unstable
debian: properly deal with Debian stable/unstable

This commit fixes the if/else block around Debian stable/unstable which
was badly merged during the last rebase.

Change-Id: Ia2eb806da3d7c8d28afffb026f35600c7125d8d7

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