[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!
We don't need to mention Python's default value here. Btw, having the empty map as the default value may be error-prone. What do you think?