[SPARK-2851] [mllib] DecisionTree Python consistency update

Added 6 static train methods to match Python API, but without default arguments (but with Python default args noted in docs).

Added factory classes for Algo and Impurity, but made private[mllib].

CC: @mengxr @dorx  Please let me know if there are other changes which would help with API consistency---thanks!
minor: We can say `Java-friendly API for [[org.apache.spark.mllib.tree.DecisionTree$#train]]` and remove the doc, so we only need to maintain a single copy of the doc.