Add an initial spec for the ssh command

[blom]

No description provided.

[pearkes]

I wonder if we can do a should_receive on the exec stub to inspect that the options are coming through clean. Unfortunately I don't for the life of me know how to properly integration test this, really.

Definitely a good start that covers the basics though.

[blom]

First I considered stubbing out Tugboat::Middleware::SSHDroplet#call entirely, but decided against it so that #call would be exercised in the tests for now. However, maybe we should stub out #call here and give it (and Kernel.exec) full attention elsewhere? Maybe this test is concerning itself too much with how #call is implemented, especially since it is testing the API call to DigitalOcean.

[pearkes]

You're right. I think for the "cli" tests this is fine - it covers what we need. I've written a handful of unit tests for the middleware, that might make sense for stubbing exec.