provider/aws: Allow cluster name, not only ARN for aws_ecs_service

[radeksimko]

Fixes #3361

Test plan

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=EcsService' 2>~/tf.log
go generate ./...
TF_ACC=1 go test ./builtin/providers/aws -v -run=EcsService -timeout 90m
=== RUN   TestAccAWSEcsServiceWithARN
--- PASS: TestAccAWSEcsServiceWithARN (34.38s)
=== RUN   TestAccAWSEcsServiceWithFamilyAndRevision
--- PASS: TestAccAWSEcsServiceWithFamilyAndRevision (32.85s)
=== RUN   TestAccAWSEcsServiceWithRenamedCluster
--- PASS: TestAccAWSEcsServiceWithRenamedCluster (133.63s)
=== RUN   TestAccAWSEcsService_withIamRole
--- PASS: TestAccAWSEcsService_withIamRole (114.89s)
=== RUN   TestAccAWSEcsService_withEcsClusterName
--- PASS: TestAccAWSEcsService_withEcsClusterName (37.41s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    353.186s

[catsby]

LGTM

[ghost]

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