DataDog forwarder cloudformation template fails in Govcloud

[carterburyNFS]

Describe what happened:
Deploying the Cloudformation template results in this error. This is in us-gov-east-1. Input only the API key from Datadog and changed the DDsite to ddog-gov.com following these instructions.

https://docs.datadoghq.com/logs/guide/forwarder/?tab=cloudformation&site=gov

Resource handler returned message: "Invalid ARN partition (Service: Iam, Status Code: 400, Request ID: e4a95305-75aa-43b9-b72f-bb3986d0bb88)" (RequestToken: 42ebb550-5208-b018-2dd6-b6fb0e5d3349, HandlerErrorCode: GeneralServiceException)

Describe what you expected:
I expected the cloudformation template to complete creating the Datadog forwarder.

Steps to reproduce the issue:

Deploy the Cloudformation template to us-gov-east-1
AWS Console will error out saying Invalid ARN partition

[fl0ge]

FYI I've created a PR (#863) because I had the same issue with AWS China.
Hopefully this will also fix Gov Cloud partition.