[SPARK-6313] Add config option to disable file locks/fetchFile cache to ...

...support NFS mounts.

This is a work around for now with the goal to find a more permanent solution.
https://issues.apache.org/jira/browse/SPARK-6313

Author: nemccarthy <nathan@nemccarthy.me>

Closes #5036 from nemccarthy/master and squashes the following commits:

2eaaf42 [nemccarthy] [SPARK-6313] Update config wording doc for spark.files.useFetchCache
5de7eb4 [nemccarthy] [SPARK-6313] Add config option to disable file locks/fetchFile cache to support NFS mounts