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.
