fix for capistrano stages with useless servers

Instances that are not running get inserted into stages even though they are not accessible, no public dns/ip, resulting in errors on deploy. Simple fix to filter for running instances only on the aws.describe_instances method.

My use case is with background workers. I have 6 worker instances with the "worker" role and at any given moment not all of them may be running. I would still like to deploy to running instances without manually updating the "worker" stage to remove empty servers.