Add support to clear-buffer-cache.py for checking SPARK_HOME for slaves

[rnowling]

trafficstars

clear-buffer-cache.py assumes the slaves file is located under /root/spark-ec2/slaves . This is not consistent with the Spark setup guide for custom deployments.

I added the ability to find the slaves file relative to the SPARK_HOME environmental variable, if set. If the slaves file cannot be found in either location, an error is raised.

[AmplabJenkins]

Can one of the admins verify this patch?

[rnowling]

Incorporated mattf's suggestion. I also updated the variables to use camel case to be consistent with the rest of the script and prioritized the EC2 file location to maintain compatibility with original version of script.