Add support for the Capistrano multistage extension

My company uses an env tag to set whether a server is a stage or production machine. I want to be able to use the Capistrano multistage to deploy to the proper set of boxes. Adding support for that through filtering the desired_instances if stage is set

- add config for aws_stages_tag to set the tag name when filtering list by stage
- add stage setting to the initialize method for CapifyEc2
- update the desired_instances method to do one of the following:
  - filter by project and stage
  - filter by project
  - filter by stage
  - all instance
- add get_instances_by_stage that take an optional instance list and filters it by stage tag
- Also my editor removes trailing whitespace, so some of the diff is just removal of trailing whitespace.