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 
1. Probably should just take a `Deployment` instance as a parameter.
2. Document parameters.
3. Open issue for error handling.