SPARK-2333 - spark_ec2 script should allow option for existing security group

    - Uses the name tag to identify machines in a cluster.
    - Allows overriding the security group name so it doesn't need to coincide with the cluster name.
    - Outputs the request id's of up to 10 pending spot instance requests.

Author: Vida Ha <vida@databricks.com>

Closes #1899 from vidaha/vida/ec2-reuse-security-group and squashes the following commits:

c80d5c3 [Vida Ha] wrap retries in a try catch block
b2989d5 [Vida Ha] SPARK-2333: spark_ec2 script should allow option for existing security group