Support additional security group option in hadoop-ec2 script

When deploying a hadoop cluster on ec2 alongside other services it is very useful to be able to specify additional (pre-existing) security groups to facilitate access control. For example one could use this feature to add a cluster to a generic "hadoop" group, which authorizes hdfs access from instances outside the cluster. Without such an option the access control for the security groups created by the script need to manually updated after cluster launch.