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.
Weird, how did I never notice that in any error message :disappointed: