Can't use parameters of type List<> for AWS::RDS::DBSubnetGroup, AWS::RDS::DBCluster and AWS::Lambda::Function.VpcConfig

[xrn]

For rds.DBSubnetGroup.SubnetIds

rds.DBSubnetGroup{
		DBSubnetGroupDescription: "Subnet Group for Aurora Cluster",
		SubnetIds:                cloudformation.Ref("SubnetIds"),
	}

rds.DBCluster.VpcSecurityGroupIds

rds.DBCluster{
		Engine:              "aurora-mysql",
                ....
		VpcSecurityGroupIds: cloudformation.Ref("SecurityGroupIds"),
	}

and lambda.function.vpcConfig.SubnetIds, lambda.function.vpcConfig.SecurityGroupIds

lambda.Function{
                ....
		VpcConfig: lambda.Function_VpcConfig{
			SubnetIds: cloudformation.Ref("SubnetIds"),
			SecurityGroupIds: [cloudformation.Ref("SecurityGroupIds"),
		}
	}

I should be able to use cloudformation.Ref but the expected type is []string

Documentation reference - https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/parameters-section-structure.html

Related to: #282

[xrn]

Example from documentation - https://d1uauaxba7bl26.cloudfront.net/latest/gzip/CloudFormationResourceSpecification.json

"AWS::RDS::DBCluster": {
      ...
      "VpcSecurityGroupIds": {
          "Documentation": "http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbcluster.html#cfn-rds-dbcluster-vpcsecuritygroupids",
          "DuplicatesAllowed": false,
          "PrimitiveItemType": "String",
          "Required": false,
          "Type": "List",
          "UpdateType": "Mutable"
        }

IMO @PaulMaddox we should think if everywhere where we can find PrimitiveItemType = string AND type = list we should replace []string with interface{} because we can use commadelieter parameters