[docker-fpm-frr]: Use supervisord as Docker entrypoint

[jleveque]

@rodnymolina: Please refactor docker-fpm-frr to use supervisord as the entrypoint in order to align the Docker with our new convention. I did not modify this Docker as I have no way to test it.

You can reference my changes to docker-fpm-quagga and docker-fpm-gobgp in PR #573 as examples.

Thanks!

[rodnymolina]

Sure Joe, will take care of this.

[stcheng]

already fixed in #2870