Create interface to fetch available slots during split calculation

HiveSplitGenerator is tightly coupled with Tez's InputContext to fetch the available slots during split calculation. Creating a interface to fetch the available slots will allow having other implementation too which are more suitable for different cloud vendors. 

The idea is to have a default implementation using Tez's InputContext and a new HiveConfiguration pointing to the class implementing the interface. Different cloud vendors can plugin there strategy by implementing the interface.