[jvm-packages] Allow supression of Rabit output in Booster::train in xgboost4j (#4262)

* Make train in xgboost4j respect print params

Previously no setting in params argument of Booster::train would prevent
the Rabit.trackerPrint call. This can fill up a lot of screen space in
the case that many folds are being trained.
* Setting "silent" in this map to "true", "True", a non-zero integer, or
  a string that can be parsed to such an int will prevent printing.
* Setting "verbose_eval" to "False" or "false" will prevent printing.
* Setting "verbose_eval" to an int (or a String parseable to an int) n
  will result in printing every n steps, or no printing is n is zero.

This is to match the python behaviour described here:
https://www.kaggle.com/c/rossmann-store-sales/discussion/17499

* Fixed 'slient' typo in xgboost4j test

* private access on two methods

jvm-packages/xgboost4j/src/test/scala/ml/dmlc/xgboost4j/scala/ScalaBoosterImplSuite.scala

@@ -139,7 +139,7 @@ class ScalaBoosterImplSuite extends FunSuite {
 
   test("cross validation") {
     val trainMat = new DMatrix("../../demo/data/agaricus.txt.train")
-    val params = List("eta" -> "1.0", "max_depth" -> "3", "slient" -> "1", "nthread" -> "6",
+    val params = List("eta" -> "1.0", "max_depth" -> "3", "silent" -> "1", "nthread" -> "6",
       "objective" -> "binary:logistic", "gamma" -> "1.0", "eval_metric" -> "error").toMap
     val round = 2
     val nfold = 5