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.
Helm3 renamed its delete command to uninstall. We should still accept
helm_command=delete for drone-helm compatibility, but the internals
should use Helm's preferred name.