Manage the RDS CA in Terraform (#50)

We're getting emails about this value needing to be bumped. The purpose of this patch is to allow us to easily revert in case something goes wrong. Therefore, this value matches what's already in production.

osem_rds/main.tf

@@ -39,4 +39,5 @@ resource "aws_db_instance" "osem" {
   storage_encrypted       = true
   vpc_security_group_ids  = [aws_security_group.osem_rds_security_group.id]
   backup_retention_period = 7
+  ca_cert_identifier      = "rds-ca-2019"
 }