AWS::Transfer::User should use `!GetAtt <Server>.ServerId` not `!Ref <Server>`

[jackvreeken]

Currently this code is generated for Users:

    TransferUser:
        Type: "AWS::Transfer::User"
        Properties:
            ServerId: !Ref TransferServer

The !Ref TransferServer should instead be !GetAtt TransferServer.ServerId.

According to the docs, the ServerId field in TransferUser expects something that matches Pattern: ^s-([0-9a-f]{17})$. According to TransferServer docs, !Ref returns something like arn:aws:transfer:us-east-1:123456789012:server/s-01234567890abcdef, whereas the GetAtt.ServerId returns something like s-01234567890abcdef.