Collect haproxy logs via syslog

Haproxy wants to log to syslog (and not stdout for performance reasons,
see https://github.com/dockerfile/haproxy/issues/3). However there is no
running syslog in our haproxy container. What we can do is mount in the
host's /dev/log and have haproxy write to the hosts syslog to get
logging.

Do this via a docker compose volume bind mount.

Change-Id: Icf4a91c2bc5f5dbb0bfb9d36e7ec0210c6dc4e90

playbooks/roles/haproxy/files/docker/docker-compose.yaml

@@ -8,6 +8,7 @@ services:
     image: haproxy:latest
     network_mode: host
     volumes:
+      - /dev/log:/dev/log
       - /var/haproxy/etc:/usr/local/etc/haproxy
       - /var/haproxy/run:/var/haproxy/run
   haproxy-statsd:

playbooks/roles/haproxy/templates/haproxy.cfg.j2

@@ -1,7 +1,7 @@
 global
   uid  1000
   gid  1000
-  log  127.0.0.1 local0
+  log  /dev/log local0
   maxconn  4000
   pidfile  /var/haproxy/run/haproxy.pid
   stats  socket /var/haproxy/run/stats uid 1000 gid 1000 mode 0600 level admin