[ML-2847] Add new tests for (DecisionTree, RandomForest)Regression, GMM, HashingTF

[smurching]

This PR follows up on #112, adding new performance tests for DecisionTreeRegression, RandomForestRegression, GMM, and HashingTF.

Summary of changes:

• Added new performance tests
• Updated configs in mllib-small.yaml
  • Alphabetized configs
  • Added new configs for: RandomForestRegression, DecisionTreeRegression, GMM, HashingTF
• Refactored TreeOrForestClassification into a trait (TreeOrForestEstimator) exposing methods for all tree/forest estimator performance tests.
  • Copied code from DecisionTreeClassification.scala into TreeOrForestEstimator.scala

I tested this PR by running the performance tests specified in mllib-small.yaml

[smurching]

This PR looks kinda big since I alphabetized the configs in mllib-small.yaml (to help keep track of which tests were being run). Let me know if it'd be helpful for me to de-alphabetize them/just commit the new test configs I've added.

[jkbradley]

LGTM I ran it locally Merging with master Thanks @smurching and @WeichenXu123 !