Configure ssh keys on all nodes using flocker-deploy

@dwgebler I've done a little bit of refactoring and added an assertion to our functional test.

Can you do a quick self-review on this PR before we submit it for external review....I'm off for lunch.

Fixes #221 
I suspect these are reversed, and that therefore the assertion is wrong as well. Note the docstring for `OpenSSHConfiguration`, which specifies `ssh_config_path` should be the *local* SSH directory.