[SPARK-5585] Flaky test in MLlib python

Add a seed for tests.

Author: Davies Liu <davies@databricks.com>

Closes #4358 from davies/flaky_test and squashes the following commits:

02371c3 [Davies Liu] Merge branch 'master' of github.com:apache/spark into flaky_test
ced499b [Davies Liu] add seed for test

python/pyspark/mllib/tests.py

@@ -285,7 +285,7 @@ class ListTests(PySparkTestCase):
         self.assertTrue(dt_model.predict(features[3]) > 0)
 
         rf_model = RandomForest.trainRegressor(
-            rdd, categoricalFeaturesInfo=categoricalFeaturesInfo, numTrees=100)
+            rdd, categoricalFeaturesInfo=categoricalFeaturesInfo, numTrees=100, seed=1)
         self.assertTrue(rf_model.predict(features[0]) <= 0)
         self.assertTrue(rf_model.predict(features[1]) > 0)
         self.assertTrue(rf_model.predict(features[2]) <= 0)