I'd like to keep Prefix's scope fairly limited, because it has potential
to spiral into something magnificently complex. You get one prefix
setting, it goes in `settings` not `environment`, end of feature.
Trying to guess in advance which part of the config a user will put in
the `settings` section and which they'll put in `environment` is a
fool's errand. Just let everything go in either place.
The ServiceAccount field only had an `envconfig` tag (as opposed to
`split_words`) because that triggered envconfig to look for the non-
prefixed form. Now that we're finding non-prefixed forms of everything,
we can use the clearer/more concise tag.
Note that TestPopulateWithConflictingVariables isn't meant to say
"here's what behavior we *want*" so much as "here's what the behavior
*is*." I don't think one thing is any better than the other, but we
should know which one we're getting.
This change revealed more about how the system needs to work, so there
are some supporting changes:
* helm.upgrade and helm.help are now vars rather than raw functions.
This allows unit tests to target the "which step should we run"
logic directly by comparing function pointers, rather than having to
configure/prepare a fully-valid Plan and then infer the logic’s
correctness based on the Plan’s state.
* configuration that's specific to kubeconfig initialization is now part
of the InitKube struct rather than run.Config, since other steps
shouldn’t need access to those settings (particularly the secrets).
* Step.Execute now receives a run.Config so it can log debug output.
