provider/aws: Add aws_kms_alias datasource

[kjmkznr]

This datasource can get arn of KMS key alias.

$  make test TEST=./builtin/providers/aws/
==> Checking that code complies with gofmt requirements...
==> Checking AWS provider for unchecked errors...
==> NOTE: at this time we only look for uncheck errors in the AWS package
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/15 03:20:50 Generated command/internal_plugin_list.go
go test -i ./builtin/providers/aws/ || exit 1
echo ./builtin/providers/aws/ | \
        xargs -t -n4 go test  -timeout=60s -parallel=4
go test -timeout=60s -parallel=4 ./builtin/providers/aws/
ok      github.com/hashicorp/terraform/builtin/providers/aws    1.671s

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccDataSourceAwsKmsAlias'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/15 03:16:14 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccDataSourceAwsKmsAlias -timeout 120m
=== RUN   TestAccDataSourceAwsKmsAlias
--- PASS: TestAccDataSourceAwsKmsAlias (68.02s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    68.031s

[stack72]

LGTM! thanks for this :)

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccDataSourceAwsKmsAlias'            ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/18 15:26:47 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccDataSourceAwsKmsAlias -timeout 120m
=== RUN   TestAccDataSourceAwsKmsAlias
--- PASS: TestAccDataSourceAwsKmsAlias (72.56s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	72.583s

[riponbanik]

using data source for kms_key_id in cloudtrail like below causes to update resource each time terraform is run
kms_key_id = "${data.aws_kms_alias.cloudtrail.arn}"

When I look at the data tfstate it shows the arn of the alias, instead of key
arn:aws:kms:ap-southeast-2:xxx:alias/xxxx

Doing as wokaround as -

arn:aws:kms:${var.aws_region}:${data.aws_caller_identity.default.account_id}:key/${data.aws_kms_alias.cloudtrail.target_key_id}

[ghost]

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