[SPARK-8467] [MLLIB] [PYSPARK] Add LDAModel.describeTopics() in Python

Could jkbradley and davies review it?

- Create a wrapper class: `LDAModelWrapper` for `LDAModel`. Because we can't deal with the return value of`describeTopics` in Scala from pyspark directly. `Array[(Array[Int], Array[Double])]` is too complicated to convert it.
- Add `loadLDAModel` in `PythonMLlibAPI`. Since `LDAModel` in Scala is an abstract class and we need to call `load` of `DistributedLDAModel`.

[[SPARK-8467] Add LDAModel.describeTopics() in Python - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-8467)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #8643 from yu-iskw/SPARK-8467-2.