F27 on StorageEncrypted for Replica DB's

[gerrievisagie]

When SourceDBInstanceIdentifier is set for AWS::RDS::DBInstance (a replica in a cluster), cf-nag (latest version) alerts with

| FAIL F27
|
| Resource: [our replica name>]
| Line Numbers: [the line number]
|
| RDS DBInstance should have StorageEncrypted enabled

see aws-resource-rds-dbinstance

This was also previously addressed in 184