Add Librato as visualization tier for collect(e)d metrics

[sopel]

This has been triggered by https://github.com/cityindex/elasticsearch-development-flow/issues/42#issuecomment-20553280. The required Librato Metrics account has been created via https://github.com/cityindex/labs-operations/issues/69 and is running as a trial still right now.

• :information_source: AWS CloudWatch metrics streaming is a readily available integration, I've enabled it already accordingly.
• :information_source: collectd integration should hopefully be easy to handle via the collectd-librato plugin.

[dpb587]

We've decided against Librato for the time being - possibly useful later for long-running instances needing real-time, aggregated monitoring.

[sopel]

See @dpb587's comment https://github.com/cityindex/logsearch-development-flow/issues/88#issuecomment-22086441 for relevant details/questions regarding the collectd-librato plugin.

[dpb587]

The PPE cluster is successfully streaming to librato; it's verified by a few custom instruments and a dashboard.