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 
If this took a `TestCase` parameter callers wouldn't have to remember to do the `addCleanup` themselves, you could do it here for them.