fix(coreos-setup-environment): Remove likely incorrect default behavior.

There is no way to ensure that this code actually produces sane defaults
and right now it may not even produce working results since networking
may not be fully configured when it runs. Instead require OEMs to
provide a coreos-setup-environment script that produces predictable
results.

This is a major change since using the $public/$private substitutions
on platforms that do not provide coreos-setup-environment will not work.