r/aws_ecs_cluster: Add arn attribute

[bflad]

Closes #2539

make testacc TEST=./aws TESTARGS='-run=TestAccAWSEcsCluster'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSEcsCluster -timeout 120m
=== RUN   TestAccAWSEcsCluster_basic
--- PASS: TestAccAWSEcsCluster_basic (10.39s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	10.425s

[Ninir]

Hi @bflad

This is looking very good, thanks for the work :) 👍

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSEcsCluster'                          
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSEcsCluster -timeout 120m
=== RUN   TestAccAWSEcsCluster_basic
--- PASS: TestAccAWSEcsCluster_basic (13.92s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	14.030s

[ghost]

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If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!