collectd: Bind-mount journal log socket

*collectd* logs to syslog, so its output is lost when it's running in a
container.  We can capture messages from it by mounting the journald
syslog socket into the container.

templates/collectd/collectd.container

@@ -7,6 +7,7 @@ Wants=network.target
 [Container]
 Image=git.pyrocufflink.net/containerimages/collectd:latest
 Volume=/:/host
+Volume=/dev/log:/dev/log:rw
 Volume=/etc/collectd.d:/etc/collectd.d:ro
 Volume=/run:/run:rw
 Tmpfs=/tmp