[SPARK-1097] Workaround Hadoop conf ConcurrentModification issue

Workaround Hadoop conf ConcurrentModification issue

Author: Raymond Liu <raymond.liu@intel.com>

Closes #1273 from colorant/hadoopRDD and squashes the following commits:

994e98b [Raymond Liu] Address comments
e2cda3d [Raymond Liu] Workaround Hadoop conf ConcurrentModification issue

(cherry picked from commit 5fa0a05763ab1d527efe20e3b10539ac5ffc36de)
Signed-off-by: Aaron Davidson <aaron@databricks.com>