Implement a callback when flowlet number of instances change

Fix for https://issues.cask.co/browse/CDAP-516.
Bamboo build: https://builds.cask.co/browse/CDAP-DUT156.
This would require documentation changes.
Because I didn't want to change the ``ProgramController`` interface and add an ``onChangeInstances`` method, and have to implement it in all the classes that implement the interface. ``command`` seemed generic enough that I could use it to do that.