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.
The tests need to allow calls to Stdout/Stderr so they don't get
"Unexpected call" errors from gomock, but these tests aren't meant to
assert that the calls actually happened. Using .AnyTimes allows 0 or
more calls.
