Description of problem:
update from ImageAsset to ReusableImageAsset for article images missed live
images which no longer had an article pointing to it.

Version-Release number of selected component (if applicable):
5.2,6.0,rickshaw

How reproducible:
problem occurs if legacy data had live article images with no current live
articles pointing to them

Fixed for 5.2 (as upgrade script) on london branch at 36382. For 6.0+, since
cms_version_map has gone away, the proper upgrade script would be this, instead:

update acs_objects 
   set object_type='com.arsdigita.cms.ReusableImageAsset' 
 where object_id in (select i.item_id
                       from cms_items i, acs_objects o
                      where i.master_id=o.object_id
                        and o.object_type='com.arsdigita.cms.ReusableImageAsset' )
   and object_type='com.arsdigita.cms.ImageAsset';