haproxy: map in config as ro

The container shouldn't need to write to its config

Change-Id: Iadbe7b663615623fbd6201aa1f2557b0eb267179

playbooks/roles/haproxy/templates/docker-compose.yaml.j2

@@ -36,7 +36,7 @@ services:
     network_mode: host
     volumes:
       - /dev/log:/dev/log
-      - /var/haproxy/etc:/usr/local/etc/haproxy
+      - /var/haproxy/etc:/usr/local/etc/haproxy:ro
       - /var/haproxy/run:/var/haproxy/run
     logging:
       driver: syslog