Throttle configurations optionally in matrix jobs

I had added an option so in Matrix jobs you can select whether the throttling it's done at parent or configuration level.

I wanted to make appear the option only when you are in a Matrix Job, but I didn't found how to do it. Any pointer will be really helpful. 

A part from that, I tested it and it works, this code is running in our Jenkins CI server. Tell me If I need to do some more changes so this can be merged.