traefik: Remove file provider config options when omitted

Currently there is an ugly error message when traefik tries to read the
dynamic_conf directory while it doesn't exists, because no dynamic
config is definied. This patch makes the config, given that it's not
defined truely optional and gets rid of the error message.

roles/traefik/templates/docker-compose.yml

@@ -105,8 +105,10 @@ services:
       - "--entryPoints.web.address=:80"
       - "--entryPoints.websecure.address=:443"
       - "--accesslog={{ traefik_access_log_enabled | bool | lower }}"
+{% if traefik_dynamic_conf != omit %}
       - "--providers.file.directory=/etc/traefik/dynamic_conf"
       - "--providers.file.watch=true"
+{% endif %}
       - "--providers.docker=true"
       - "--providers.docker.endpoint=tcp://dockersocket:2375"
       - "--providers.docker.exposedByDefault=false"