Set spark.executor.uri from environment variable (needed by Mesos)

The Mesos backend uses this property when setting up a slave process.  It is similarly set in the Scala repl (org.apache.spark.repl.SparkILoop), but I couldn't find any analogous for pyspark.

Author: Ivan Wick <[email protected]>

This patch had conflicts when merged, resolved by
Committer: Matei Zaharia <[email protected]>

Closes apache#311 from ivanwick/master and squashes the following commits:

da0c3e4 [Ivan Wick] Set spark.executor.uri from environment variable (needed by Mesos)

Author: ivanwick

python/pyspark/shell.py

@@ -29,6 +29,9 @@
 # this is the equivalent of ADD_JARS
 add_files = os.environ.get("ADD_FILES").split(',') if os.environ.get("ADD_FILES") != None else None
 
+if os.environ.get("SPARK_EXECUTOR_URI"):
+    SparkContext.setSystemProperty("spark.executor.uri", os.environ["SPARK_EXECUTOR_URI"])
+
 sc = SparkContext(os.environ.get("MASTER", "local[*]"), "PySparkShell", pyFiles=add_files)
 
 print """Welcome to