[SPARK-6506] [pyspark] Do not try to retrieve SPARK_HOME when not needed...

....

In particular, this makes pyspark in yarn-cluster mode fail unless
SPARK_HOME is set, when it's not really needed.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5405 from vanzin/SPARK-6506 and squashes the following commits:

e184507 [Marcelo Vanzin] [SPARK-6506] [pyspark] Do not try to retrieve SPARK_HOME when not needed.

python/pyspark/java_gateway.py

@@ -29,11 +29,10 @@ from pyspark.serializers import read_int
 
 
 def launch_gateway():
-    SPARK_HOME = os.environ["SPARK_HOME"]
-
     if "PYSPARK_GATEWAY_PORT" in os.environ:
         gateway_port = int(os.environ["PYSPARK_GATEWAY_PORT"])
     else:
+        SPARK_HOME = os.environ["SPARK_HOME"]
         # Launch the Py4j gateway using Spark's run command so that we pick up the
         # proper classpath and settings from spark-env.sh
         on_windows = platform.system() == "Windows"