fixes #7331 - delete unassigned os default templates

**The problem**: Previously assigned config template is not deleted when unassigning it. Therefore, can never delete the config template (the error will raise that it belongs to os_default_template).
**The fix**: adds ```ruby :_destroy => 1``` to existing os_default_template, if the config_template_id is blank.
@domcleal @eLobato - I have removed 
as we escape HTML. to_sentence method will display them nicer imho