Fixes #8514 - Add support for requests to smart proxy discovery plugin

I don't mind changing it.
I just wanted the `belongs_to` to look like the rest of them in subnet:
``` ruby
 belongs_to :dhcp, :class_name => "SmartProxy"
 belongs_to :tftp, :class_name => "SmartProxy"
 belongs_to :dns,  :class_name => "SmartProxy"
```