Improvement for upstream ssl_type

#### Problem description

Currently user needs to modify `docker-compose.yml` if they want to use upstream SSL. I think we should pass `UPSTREAM_HTTPS_PORT` and `SSL_TYPE` environment variables to the [start-odk.sh](https://github.com/getodk/central/blob/master/files/service/scripts/start-odk.sh) and [start-enketo.sh](https://github.com/getodk/central/blob/master/files/enketo/start-enketo.sh). So that `BASE_URL` can be decided accordingly.