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.
Few more things that might make this a bit nicer (but will probably break some tests) ((also optional)):

1)  We should really make everything but the `initialize` and `execute` methods private.  Those are the only two that are really part of the public API of this class.  This will probably break some tests.
2)  Even with my suggestion for a refactor of your method, the `.api_call` portion of things is a bit long.  You could arguably move that to a separate method like `def request_instances`.  Might be a bit overkill, so your call.