Enable changing username and database name in rds-broker

[rbogle]

In order to fully restore a deleted database service instance for customers, we want to provide them a transparent restoration of the old db to the new db including the same credentials and database name.

Acceptance Criteria

• GIVEN an operator is restoring an AWS rds snapshot to a new broker service instance
  AND the aws-broker stores a new password, username, and database name for the service instance
  WHEN we restore the snapshot and redirect the new service instance to the restored instance
  AND the username and database name no longer matches between the new and old
  THEN the operator should be able to replace the new username and database name in the broker with the restored ones
  SO that the service key credentials provided by the service broker match the credentials and connection info required to connect to the RDS instance in AWS.

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Security considerations

Will improve and complete our ability to fully restore a tenant database when it has been completely deleted.

Implementation sketch

[links to background notes, sketches, and/or relevant documentation

• Update the rds broker to have a field specifically for database name separate from the database instance name
• Update the rds broker to fill in the database name when none exists with the default based on the database instance name (aws)
• Update the rds broker to provide the database name as distinct from the instance name (aws) in the name field of the credentials
• Bundle a script in the broker that can be run by an operator to change the username, database name for an rds instance.
• Document using this script in the rds restoration runbook. https://github.com/cloud-gov/internal-docs/blob/main/docs/runbooks/AWS/restoring-rds.md