Added "arn" attribute to AWS Lambda alias

[tmshn]

What has changed?

Per title.

Why it's needed?

Without the arn attribute, you have to combine function_name and name to specify arn, which is very troublesome.
For instance,

resource "aws_cloudwatch_event_target" "my_event_target" {
  rule = "my-event-rule"
  target_id = "tf-my-event-target"
  arn = "${aws_lambda_alias.my_alias.arn}:${aws_lambda_alias.my_alias.name}"
}

[apparentlymart]

$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSLambdaAlias_basic"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/10/16 10:00:27 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSLambdaAlias_basic -timeout 120m
=== RUN   TestAccAWSLambdaAlias_basic
--- PASS: TestAccAWSLambdaAlias_basic (19.70s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    19.717s

[apparentlymart]

Thanks @tmshn! This looks great.

[ghost]

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