SPARK-1532 Provide option in ec2/spark_ec2.py to set more restrictive Source in the security groups.

SPARK-1532 add option to ec2/spark_ec2.py to permit more restrictive IP address to EC2 instance.

The new option is --ips-allowed and it defaults to 0.0.0.0/0 to
match the existing behavior of ec2/spark_ec2.py