Start stop containers 252

Not ready for review.
You probably don't need to worry too much about it in this branch. The configuration should already have been validated by `flocker-deploy` on the user's laptop. That's where we need to ensure they get meaningful error messages.

But the condition which leads to the `TypeError` should probably be handled in `model_from_config` and turned into a meaningful `ConfigurationError` message there. Raise a ticket for that condition.