add plugin cfn_output

[fujiwara]

This PR adds cfn_output template function to lookup cloudformation stack's output.

For example,

# config.yaml
# ...
service_definition: service-def.json
plugins:
  - name: cfn_output

aws cloudformationcfn describe-stacks --stack-name EcsSampleStack output.

{
  "Stacks": [
    {
      "StackId": "arn:aws:cloudformation:ap-northeast-1:012345678901:stack/EcsSampleStack/790004d0-42aa-11eb-8fb1-06aa9b7dfc56",
      "StackName": "EcsSampleStack",
      "Outputs": [
        {
          "OutputKey": "Subnet1Output",
          "OutputValue": "subnet-01036767a9c7c3ab8"
        },
        {
          "OutputKey": "SecurityGroupIdOutput",
          "OutputValue": "sg-0aa077022a7a4c54d"
        },
        {
          "OutputKey": "Subnet2Output",
          "OutputValue": "subnet-0890a957951b89645"
        }
      ]
    }
  ]
}

service-def.json

{
  "networkConfiguration": {
    "awsvpcConfiguration": {
      "assignPublicIp": "ENABLED",
      "securityGroups": [
        "{{ cfn_output `EcsSampleStack` `SecurityGroupIdOutput` }}"
      ],
      "subnets": [
        "{{ cfn_output `EcsSampleStack` `Subnet1Output` }}",
        "{{ cfn_output `EcsSampleStack` `Subnet2Output` }}"
      ]
    }
  }
}

ecspreso render --service-definition outputs as below.

{
  "networkConfiguration": {
    "awsvpcConfiguration": {
      "assignPublicIp": "ENABLED",
      "securityGroups": [
        "sg-0aa077022a7a4c54d"
      ],
      "subnets": [
        "subnet-01036767a9c7c3ab8",
        "subnet-0890a957951b89645"
      ]
    }
  }
}

[fujiwara]

continue in #217.