[SPARK-8576] Add spark-ec2 options to set IAM roles and instance-initiated shutdown behavior

Both of these options are useful when spark-ec2 is being used as part of an automated pipeline and the engineers want to minimize the need to pass around AWS keys for access to things like S3 (keys are replaced by the IAM role) and to be able to launch a cluster that can terminate itself cleanly.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #6962 from nchammas/additional-ec2-options and squashes the following commits:

fcf252e [Nicholas Chammas] PEP8 fixes
efba9ee [Nicholas Chammas] add help for --instance-initiated-shutdown-behavior
598aecf [Nicholas Chammas] option to launch instances into IAM role
2743632 [Nicholas Chammas] add option for instance initiated shutdown