Spark Integration: Collect System Metrics from Spark Job

Build running here: https://builds.cask.co/browse/CDAP-DUT176-5

@anew : Please review. Thanks. 
'u' 's' and others are spread all over the codebase. I have tried to change it wherever possible but can't do all of them in this PR. 
I will do the changes if needed in a different PR with after user metrics PR is also merged