Spark now uses configure style

- Spark uses configurer style API : CDAP-382

Build running here: https://builds.cask.co/browse/CDAP-DUT316-2

This opens two more related (but independent to this merge) issues: 
CDAP-989: Doc changes for the Spark configurer style API
CDAP-990: Apps and Guide changes to use Spark configurer style API
You can turn this interface into a concrete class by merging with the `DefaultSparkSpecification`. Also, one of the goal is to remove guava dependency from cdap-api classes, hence try to remove usage of guava as well (e.g. instead of `ImmutableMap.copyOf`, use `new HashMap....`).