Change StepsRunner to be more of a builder to prepare for assigning Allocations.

[coollog]

Part of #1297

This defers running the steps till the end and allows for StepsRunner to know how many steps there are before running them all.

[chanseokoh]

IMHO, in this case, I think it's easier to understand when passing StepsRunner instead of a (basically) Callable (I mean ImageBuildRunnable). Using Callable makes sense if you need to invoke some logic later, but knowing that by design having a StepsRunner instance does not kick off running its steps, it is sort of like StepsRunner embeds a Callable in itself, so I think it's OK to have a separate "run" method in StepsRunner (like the previous runStepsRunners()). I think this will almost feel like StepsRunner is implementing Callable. Then BuildSteps could just call the "run" method.