Improve aws rds_instance default security posture

[azerella]

The Terraform module aws_db_instance provides an argument of storage_encrypted which is defaulted to off when not specified. I think it would be a good idea if the https://github.com/alphagov/govuk-aws/blob/master/terraform/modules/aws/rds_instance/main.tf module here; specified this argument and defaulted to true.

Setting the storage_encrypted argument will require a kms_key_id to be used as mentioned here: https://www.terraform.io/docs/providers/aws/r/db_instance.html

Encrypted RDS instances shows an improved security posture in the event that the vender's RDS service is compromised. Additionally, AWS don't provide a easy solution for encrypting already unencrypted RDS instances without wiping data, as a default an RDS instance should be encrypted.