CNG: Improve `set-config` to enable transition to gomplate

Summary

The set-config script should be expanded to allow for the use of gomplate in parallel to ERB, as a transitional step between the two configuration paradigms.

Details

Currently, /scripts/set-config is called by the /scripts/entrypoint.sh of gitlab-ruby originated service images within the CNG (currently, almost all). We need to be able to operate in both ERB and gomplate simultaneously during development. As such, we should expand the functionality of gitlab-rub/scripts/set-config to be capable of calling out to gomplate when such recognized.