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
@tstrachota, I removed here your suggestion of 
```ruby
 `render_error` error_msg, :status => :unprocessable_entity
```

since `error_message` is not a template name like `not_found`, `unprocessable_entity`.  If I use `unprocessable_entity`, I don't get the `error_msg` as the response that I want.
I added `:layout => 'api/v2/layouts/error_layout' instead and removed {:error => ...} which is not reflected in the tests.