This pull request amends the broken #3062 allow Spark 2.2 to work.
Please note this won't work in Spark <=2.1 as sc.removeSparkListener was implemented in Spark 2.2. (So perhaps a more general method is better, although that is what was attempted in #3062)
This PR fixes: #3208, #3151 and the discussion in #1927.
I do find it strange that #3062 dose not work in Spark 2.2, it's probably due to some sort of public/private issue in the org.apache.spark.scheduler.LiveListenerBus class inheritance (In Spark itself). The error is: `java.lang.NoSuchMethodError: org.apache.spark.scheduler.LiveListenerBus.removeListener(Ljava/lang/Object;)V`
* add back train method but mark as deprecated
* add back train method but mark as deprecated
* fix scalastyle error
* fix scalastyle error
* consider spark.task.cpus when controlling parallelism
* fix bug
* fix conf setup
* calculate requestedCores within ParallelismController
* enforce spark.task.cpus = 1
* unify unit test case framework
* enable spark ui
* add back train method but mark as deprecated
* add back train method but mark as deprecated
* fix scalastyle error
* fix scalastyle error
* update resource files
* Update SparkParallelismTracker.scala
* remove xgboost-tracker.properties
* Add SparkParallelismTracker to prevent job from hanging
* Code review comments
* Code Review Comments
* Fix unit tests
* Changes and unit test to catch the corner case.
* Update documentations
* Small improvements
* cancalAllJobs is problematic with scalatest. Remove it
* Code Review Comments
* Check number of executor cores beforehand, and throw exeception if any core is lost.
* Address CR Comments
* Add missing class
* Fix flaky unit test
* Address CR comments
* Remove redundant param for TaskFailedListener
