Allow override of default statsd output port/host

Make the port specifiable, as we are currently migrating to the
use of Bucky on udp/8126.

Makes sense to allow override of the host too.
Fix issue with logstash restarting

A bug was introduced in d722af2df65d835dd35bdf33f2cdf3d74071e171
where logstash service would not restart after config change.
Update CHANGELOG
Merge pull request #35 from ministryofjustice/statsd_port

Allow custom statsd port and host (for migration back to Bucky on 8126); fix restart on config update