Ensuring a single parent for resources with multiple parents

[a2ush]

Issue #, if available: related #9

If a resource has multiple parents (e.g. a TGWAttachment related to both a VPC and a Subnet), the resource is not rendered.

Resources:
## VPC and Subnet definitions are omitted.
  VPCAattachment:
    Type: AWS::EC2::TransitGatewayAttachment
    Properties:
      SubnetIds:
        - !Ref SubnetA
      TransitGatewayId: !Ref TGW
      VpcId: !Ref VPCA

  VPCBattachment:
    Type: AWS::EC2::TransitGatewayAttachment
    Properties:
      SubnetIds:
        - !Ref SubnetB
      TransitGatewayId: !Ref TGW
      VpcId: !Ref VPCB

Description of changes:

• If there is a parent-child relationship among the children of a resource ( a resource is their grandparent), it is deleted from grandparent's children resource.

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