Add new AWS::RDS::DBCluster VALID_DB_ENGINE_MODES (#1573)
Add new AWS::RDS::DBCluster VALID_DB_ENGINE_MODES (#1573)

- Add new AWS::RDS::DBCluster VALID_DB_ENGINE_MODES
- Fixes #1572
ikben/troposphere,cloudtools/troposphere,cloudtools/troposphere,ikben/troposphere