Explicitly set the syslog server host

Explicit is better than implicit
Set logstash up to listen for syslog messages

This involves making sure that the monitoring machine is not acting as
an rsyslog server and instead there is a logstash input that listens to
the syslog port.
Merge pull request #93 from alphagov/put-syslog-in-logstash

Put syslog in logstash