Prevent module shadowing in pyspark_runner.py (#2232)
Prevent module shadowing in pyspark_runner.py (#2232)

PySparkTask uses spark-submit to run the script pyspark_runner.py.
Because it is run as a script the modules and packages from its
directory (luigi/contrib/) shadow the global modules. In particular the
package `hdfs` which is used by webhdfs_client.py is shadowed by the
`luigi.contrib.hdfs` package. Thus PySparkTask does not work together
with webhdfs.

The problem is resolved by putting the current directory at the end of
the path list `sys.path`.
Magnetic/luigi,republic-analytics/luigi,riga/luigi,spotify/luigi,rayrrr/luigi,Magnetic/luigi,Magnetic/luigi,adaitche/luigi,spotify/luigi,soxofaan/luigi,foursquare/luigi,spotify/luigi,PeteW/luigi,edx/luigi,riga/luigi,riga/luigi,PeteW/luigi,rayrrr/luigi,Houzz/luigi,foursquare/luigi,republic-analytics/luigi,Houzz/luigi,edx/luigi,dlstadther/luigi,republic-analytics/luigi,adaitche/luigi,samuell/luigi,rayrrr/luigi,adaitche/luigi,samuell/luigi,PeteW/luigi,edx/luigi,samuell/luigi,jamesmcm/luigi,dlstadther/luigi,republic-analytics/luigi,Houzz/luigi,dlstadther/luigi,foursquare/luigi,spotify/luigi,Magnetic/luigi,soxofaan/luigi,Houzz/luigi,foursquare/luigi,dlstadther/luigi,adaitche/luigi,riga/luigi,jamesmcm/luigi,jamesmcm/luigi,jamesmcm/luigi,edx/luigi,soxofaan/luigi,samuell/luigi,soxofaan/luigi,rayrrr/luigi,PeteW/luigi