provider/aws: aws_elasticache_cluster data source

[stack72]

Fixes #11445

% make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccAWSDataElasticacheCluster_'                                         2 ↵ ✹ ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/29 00:01:59 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccAWSDataElasticacheCluster_ -timeout 120m
=== RUN   TestAccAWSDataElasticacheCluster_basic
--- PASS: TestAccAWSDataElasticacheCluster_basic (508.45s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	508.479s

[catsby]

LGTM

[ghost]

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