automated_backups_enabled should not fail for RDS Postgres replicas

[jcaffet]

Hi,
For rds service, the automated_backups_enabled check fails when "Automated backup is not enabled"
If is true for RDS master instances, but it should not be the case for RDS Postgres replicas where backups are not supported : https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.html

Regards