Replace two methods with a single one that returns the cert. This moves more
of the logic into the single caller (auto-config).
tlsutil.Configurator is widely used. By keeping it smaller and focused only on storing and
returning TLS config, we make the code easier to follow.
These two methods were more related to auto-config than to tlsutil, so reducing the interface
moves the logic closer to the feature that requires it.
R.B. Boyer