Allow more complex `up --init` dependencies

[emk]

I'm seeing some cases where users need to write:

cage up --init
cage up some_pod --init
cage up

I'd like to reduce that to a single cage up --init, but that may require some way of declaring more complex dependencies between pods. I think the current mechanism is "placeholders start first, then all other pods start in parallel."

Maybe this already works using that rule? Anyway, check it.