SSH only should return running instances

This is a fix for [this bug](https://github.com/sportngin/opsicle/issues/63)


Rollback Plan
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* To roll back this change, revert the merge with: `git revert -m 1 MERGE_SHA` and perform another deploy.

URLs
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https://github.com/sportngin/opsicle/issues/63

QA Plan
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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.