Expose the Default Traefik Log Level variable

Instead of hiding it in the documentation and in the
docker-compose.acme.yml and docker-compose.traefik.yml files, adding it
to the sample.env file

sample.env

@@ -35,6 +35,7 @@ PROJECT_DRUPAL_DOCKERFILE=Dockerfile
 # Includes `traefik` as a service, if false assume we are sharing a traefik
 # from another project.
 INCLUDE_TRAEFIK_SERVICE=true
+TRAEFIK_LOG_LEVEL=ERROR
 
 # Should we use ACME to generate a SSL Certificate
 USE_ACME=false