autoscaling: Support passing the IAMInstanceProfile through to a launch configuration

References:

 - https://github.com/hashicorp/terraform/issues/323
 - https://github.com/mitchellh/goamz/issues/73

It wasn't clear to me how to write a test for this, since the instance profile has to exist for you to reference it. If you have any feedback on that, I'd be happy to implement the test too.