Spark 1532- provide option to specify allowed IPs for EC2 instance when creating EC2 cluster

Modified the python script that creates spark clusters on EC2.  Instead of always allowing '0.0.0.0/0' to access the open ports, the script now provides the option to set the allowed IPs.  It defaults to '0.0.0.0/0' so only users who actually specify the new option will see a change in behavior.

https://issues.apache.org/jira/browse/SPARK-1532