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.
I'd remove parentheses and call variables ```template_exists``` and ```template_empty```, it took me some time to figure out the meaning.