Physical resource ID returned to custom resource ought to be the domain name

[erincerys]

It'd appear that when this was written, the DomainId was thought to be the proper physical resource ID to return, and as such, the return was modeled after that. Built-in resource AWS::Elasticsearch::Domain returns its physical resource ID as the domain name. This can be referenced to e.g. construct ARNs in policies. This function returns <accountId>/<domainName> as the physical resource ID for the custom resource on creation. This causes references to the custom resource in CFN to produce unexpected results and errors. I've submitted a PR to have the custom resource's physical resource ID be consistent with what the equivalent build-in ES domain resource returns.