provider/aws: Add IPv6 outputs to aws_subnet datasource

[stack72]

Fixes: #13595

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccDataSourceAwsSubnet'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/21 13:52:43 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccDataSourceAwsSubnet -timeout 120m
=== RUN   TestAccDataSourceAwsSubnetIDs
--- PASS: TestAccDataSourceAwsSubnetIDs (81.05s)
=== RUN   TestAccDataSourceAwsSubnet
--- PASS: TestAccDataSourceAwsSubnet (57.48s)
=== RUN   TestAccDataSourceAwsSubnetIpv6ByIpv6Filter
--- PASS: TestAccDataSourceAwsSubnetIpv6ByIpv6Filter (82.63s)
=== RUN   TestAccDataSourceAwsSubnetIpv6ByIpv6CidrBlock
--- PASS: TestAccDataSourceAwsSubnetIpv6ByIpv6CidrBlock (82.43s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	303.625s

[grubernaut]

LGTM

[ghost]

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