[FIX] do not load defaults when testing SparkConf in pyspark

The default constructor loads default properties, which can fail the test.

Author: Xiangrui Meng <meng@databricks.com>

Closes #775 from mengxr/pyspark-conf-fix and squashes the following commits:

83ef6c4 [Xiangrui Meng] do not load defaults when testing SparkConf in pyspark

python/pyspark/conf.py

@@ -33,7 +33,7 @@ u'My app'
 >>> sc.sparkHome == None
 True
 
->>> conf = SparkConf()
+>>> conf = SparkConf(loadDefaults=False)
 >>> conf.setSparkHome("/path")
 <pyspark.conf.SparkConf object at ...>
 >>> conf.get("spark.home")