Provider AWS: Add Placement Strategy to aws_ecs_service resource

[gthole]

Add placement_strategy on ECS Service Creation (modification or removal require recreation) to complement feature support for placement_constraints recently added to Task Definitions (from which much of this PR is derived).

[stack72]

LGTM! Thanks for the work here @gthole :)

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSEcsService'                                                          ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/17 17:46:26 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSEcsService -timeout 120m
=== RUN   TestAccAWSEcsServiceWithARN
--- PASS: TestAccAWSEcsServiceWithARN (115.09s)
=== RUN   TestAccAWSEcsServiceWithFamilyAndRevision
--- PASS: TestAccAWSEcsServiceWithFamilyAndRevision (111.34s)
=== RUN   TestAccAWSEcsServiceWithRenamedCluster
--- PASS: TestAccAWSEcsServiceWithRenamedCluster (217.37s)
=== RUN   TestAccAWSEcsService_withIamRole
--- PASS: TestAccAWSEcsService_withIamRole (127.86s)
=== RUN   TestAccAWSEcsService_withDeploymentValues
--- PASS: TestAccAWSEcsService_withDeploymentValues (123.62s)
=== RUN   TestAccAWSEcsService_withLbChanges
--- PASS: TestAccAWSEcsService_withLbChanges (236.25s)
=== RUN   TestAccAWSEcsService_withEcsClusterName
--- PASS: TestAccAWSEcsService_withEcsClusterName (111.28s)
=== RUN   TestAccAWSEcsService_withAlb
--- PASS: TestAccAWSEcsService_withAlb (137.32s)
=== RUN   TestAccAWSEcsServiceWithPlacementStrategy
--- PASS: TestAccAWSEcsServiceWithPlacementStrategy (212.95s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	1393.122s

[ghost]

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