Support opt-in of shipping RDS logs to Kibana

[spgreenberg]

In order to support customers needing RDS logs in external (non cloud.gov) systems, customers with standalone RDS instances need to be able to opt-in to log shipping to S3.

The scope of this story is only to opt in to the configuration. It is does not encompass shipping logs.

Acceptance Criteria

• GIVEN a customer with a dedicated RDS instance (not shared)
  WHEN the customer needs logs shipped to S3
  THEN the customer can opt-in \

Security considerations

None. This should just be a configuration setting in the broker DB. The scope of this story is only to opt-in to the configuration. It does not encompass shipping logs.

Implementation sketch

• Enable a configuration setting to be passed to the broker on service instance creation or update

[markdboyd]

Superseded by

#372
https://github.com/cloud-gov/private/issues/957