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.
```java
Retry(maxAttempts = 5 )
```
Will look better than just having number. In the future, if we want to extend based on other dimensions - ex: duration we can extend this API.