Add elasticsearch-curator cron job to purge old logstash database

Elasticsearch curator is a contrib package provided by Elasticsearch
that helps housekeep the Logstash data stored. By default we keep
curation disabled.

More docs on: https://pypi.python.org/pypi/elasticsearch-curator
changelog for elasticsearch-curator
Merge pull request #8 from ministryofjustice/curator

Add elasticsearch-curator cron job to purge old logstash database