Flocker deploy ssh config 71

Fixes #71 
The Conch SSH server needs to get a chance to run.  Since `configure_ssh` is a blocking API (and trial runs tests in the reactor thread) if we don't use `deferToThread` here then the SSH server never gets a chance to run and the test hangs (since `configure_ssh` cannot complete unless the SSH server is talking to it - a deadlock, essentially).