SPARK-1790: Update EC2 scripts to support r3 instance types

Author: Varakhedi Sujeet <svarakhedi@gopivotal.com>

Closes #960 from sujeetv/ec2-r3 and squashes the following commits:

3cb9fd5 [Varakhedi Sujeet] SPARK-1790: Update EC2 scripts to support r3 instance
(cherry picked from commit 11ded3f66f178e4d8d2b23491dd5e0ea23bcf719)

Conflicts:

	ec2/spark_ec2.py