-a/--cascade-affected only cascades to _existing_ containers

Follow-up to https://github.com/michaelsauter/crane/pull/69

The main (only?) use-case for `-a/--cascade-affected` is to select containers that would be _affected_ by a change in the state of the explicitly-targeted containers. Thus, it does not make sense to select the ones that have not been created (and thus never been linked/volumesFrom'ed/network-linked).

The behavior for `-d/--cascade-dependencies` remains unchanged (tests ensuring that), i.e. that all containers declared in the config that the explicitly-targeted containers depend on, whether they exist or not, are added to the target. I think this behavior maps nicely to the current flag names, as `dependencies` is not as runtime-specific as `affected`.

Also included, a first attempt at mocking, utilizing b6963aa1747f55485bc63879ff64e50e77decc20.