fixes #5780 - API v2 - RESTful way to add/remove many-to-many association one at a time rather than passing all id's

Note: Related to PR https://github.com/theforeman/foreman/pull/1773 to add auditing when adding/removing
I think we should do 
```ruby 
render_error error_msg, :status => :unprocessable_entity
``` 
here to use the rabl template and keep the response structure consistent.