improve certbot configuration and documentation

[kwypchlo]

It should be easier for portal operators to use different certbot dns plugin ie. cloudflare.

List of all dns plugins: https://hub.docker.com/r/certbot/certbot

certbot:
  image: certbot/dns-route53:v1.25.0
  entrypoint: sh /entrypoint.sh
  container_name: certbot
  restart: unless-stopped
  logging: *default-logging
  env_file:
    - .env
  environment:
    - CERTBOT_ARGS=--dns-route53
  volumes:
    - ./docker/certbot/entrypoint.sh:/entrypoint.sh
    - ./docker/data/certbot:/etc/letsencrypt

Right now, to replace dns-route53 plugin with dns-cloudflare portal operator needs to make sure cloudflare api key is available in the env file and add this to docker-compose.override.yml manually:

certbot:
  image: certbot/dns-cloudflare:v1.25.0
  environment:
    - CERTBOT_ARGS=--dns-cloudflare

We would like to be able to use a config to select the plugin and let ansible generate proper override file entry.