Add c3 instance types to Spark EC2

The number of disks for the c3 instance types taken from here: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html#StorageOnInstanceTypes

Author: Christian Lundgren <christian.lundgren@gameanalytics.com>

Closes #595 from chrisavl/branch-0.9 and squashes the following commits:

c8af5f9 [Christian Lundgren] Add c3 instance types to Spark EC2
(cherry picked from commit 19b4bb2b444f1dbc4592bf3d58b17652e0ae6d6b)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>