This fixes the run package's leaky abstraction; other packages no longer
need to know or care that run.Config even exists.
Note that since the various Steps now depend on having a non-nil pointer
to a run.Config, it's unsafe (or at least risky) to initialize them
directly. They should be created with their NewSTEPNAME functions. All
their fields are now private, to reflect this.
This is the first step toward removing run.Config entirely. InitKube was
the only Step that even used cfg in its Execute function; the rest just
discarded it.
This seems to be be a more natural separation of concerns--the knowledge
of which config fields map to which parts of a Step belong to the Step,
not to the Plan.
