SPARK-4767: Add support for launching in a specified placement group to spark_ec2

Placement groups are cool and all the cool kids are using them. Lets add support for them to spark_ec2.py because I'm lazy

Author: Holden Karau <holden@pigscanfly.ca>

Closes #3623 from holdenk/SPARK-4767-add-support-for-launching-in-a-specified-placement-group-to-spark-ec2-scripts and squashes the following commits:

111a5fd [Holden Karau] merge in master
70ace25 [Holden Karau] Placement groups are cool and all the cool kids are using them. Lets add support for them to spark_ec2.py because I'm lazy