fixes #1646, #3103 - enable cloning for templates, and add read-only options

Good question :grin:  

It's assuming that each taxonomic model has a host association (?), and this blows up for config templates.  I tried returning [] if Host::Base.column_names didn't include the keys in taxonomy_foreign_key_conditions, but that caused the Smart Proxy taxonomy tests to explode...

Any ideas what I should do? I'm not even really sure what these used_???_ids methods are supposed to be doing.