[WIP] Integrate `Sensu` with Growthforecast.

This commit is intended to integrate `Sensu` with `Growthforecast`.
Add `Sensu` metrics configurations.

- Deploy the scripts for collecting metrics.
- Add `Sensu` metrics configurations.
Add `Sensu` mutator configuration.

- Deploy the mutator script
- Add `Sensu` mutator configuration.
Add `Sensu` handler configuration for `Growthforecast`.

- Install `Growthforecast` gem.
- Deploy the handler script.
- Add `Sensu` handler configuration for `Growthforecast`.
Add tests for integrating `Sensu` with `Growthforecast`.
Merge pull request #139 from kazu634/sensu-growthforecast-integration

Integrate `Sensu` with Growthforecast.