Updated cluster_test.py

1. Updated paths to spark-shell and pyspark 
2. Enumerating mount by doing 'ls /' is buggy as there may be directories /mnt* that are not really mount points and should not be tested for usage by ephemeral HDFS. Instead this enumerates the actual mount points, and tests only those for usage by ephemeral HDFS.
Merge pull request #1 from tdas/patch-1

Updated cluster_test.py