installer: Print substitution values too

Not sure why we weren't, looks like a bug.
installer: Rename boot.iso -> installer.iso afterwards again

This code used to exist but was dropped.  Reinstate it, as it's
important to distinguish that the content is included.
imagefactory: Fix bug OZ config memory parsing

This patch fixes a bug where if the memory option does not
exist in the libvirt section of the oz.cfg, it produced a
traceback.  We now check to see if it exists, before attempting
to work with it.  Additionally, we now use the ConfigParser method
to process the oz configuration file, as is more inline with the
rest of -toolbox.