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

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.
suggested refactor - lift the test out of the if spark_home is None block, use a generic message