Nothing exists in the plain vanilla version of the Selenium Grid.
But that doesn't mean you cant build any of these on your own.
Some of these are already available as part of the Enhanced Grid that we built for Selion
So using the SeLion Grid, you should be able to start/shutdown nodes at will.
For the other things :
Making changes in node config.json <<<<<< What is a typical use case for this ?
Showing health of a node <<<<<< What do you mean by health of a node ? Are you hinting at the machine vitals ? If yes, then you should be able to build a servlet and inject it into the Hub which would have access to all the nodes information and which would invoke another servlet (this would be injected into the node) and get this information for you.
For more information, you can browse through the following: