Add RDS engine type "oracle-ee-cdb"

I notice that in `troposphere/validators/rds.py` the `VALID_DB_ENGINES` list is probably missing `oracle-ee-cdb` as a possible option.

https://github.com/cloudtools/troposphere/blob/main/troposphere/validators/rds.py#L87

Where I work, we have clients that use that option for deploying Oracle databases on AWS.