Datadog integrations

This will add datadog integrations for:
- chef
- basic agent
- nginx (app only, since the balancer nginx doesn't have the status module)
- memcached
- redis

Postgres integration will need a user created - I didn't add that yet.

Also, you will need to add an item to the `secrets` databag with this structure:

```json
{
  "id": "datadog",
  "api_key": "xxxxxxx",
  "application_key": "xxxxxxx"
}
```

/cc @skottler