feat(contrib/ec2): add optional support for IAM instance profiles

This supersedes #2539 by fixing/extending a5119b1 to allow the IAM instance profiles (more frequently referred to as [IAM roles for Amazon EC2](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html)) to be optional - it is implemented by means of [conditional CloudFormation resources](http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-conditions.html), see https://github.com/deis/deis/pull/2539#issuecomment-64896466 for more details.

I've tested the resulting template with and without specifying an IAM instance profile and the Deis cluster has been created successfully for both variations.