SSH only should return running instances

This is a fix for [this bug](https://github.com/sportngin/opsicle/issues/63)


Rollback Plan
-------------
* To roll back this change, revert the merge with: `git revert -m 1 MERGE_SHA` and perform another deploy.

URLs
----
https://github.com/sportngin/opsicle/issues/63

QA Plan
-------
Create a .opsicle file for a stack with some stopped instances. Run `bundle exec opsicle ssh {environment}` and make sure stopped instances don't show up.
You could trim off a line as well by doing something like this:

```ruby
@instances ||= client.api_call(:describe_instances, { stack_id: client.config.opsworks_config[:stack_id] })
                     .data[:instances]
                     .reject { |instance| instance[:status].to_s != 'online'}
```