Logrotate graphite access log

Access logs up to 6gb on preview, also big in production and staging.
The problem file is /opt/graphite/storage/access.log, /opt/graphite/storage/error.log could also be a problem.
The motivation came from low disk space warnings on pp-preview.
Merge pull request #298 from alphagov/rotate-graphite-access-log

Logrotate graphite access log