[SPARK-15092][SPARK-15139][PYSPARK][ML] Pyspark TreeEnsemble missing methods

## What changes were proposed in this pull request?

Add `toDebugString` and `totalNumNodes` to `TreeEnsembleModels` and add `toDebugString` to `DecisionTreeModel`

## How was this patch tested?

Extended doc tests.

Author: Holden Karau <holden@us.ibm.com>

Closes #12919 from holdenk/SPARK-15139-pyspark-treeEnsemble-missing-methods.