provider/aws: Set aws_subnet ipv6_cidr_block to computed

[stack72]

Fixes: #14361

An IPv6 CIDR block is option and can be added after a subnet has been
created. Therefore, we should set it to Computed: true

Otherwise, a manually created IPv6 association will be removed on the
next terraform run

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSSubnet_'                      
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 17:16:15 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSSubnet_ -timeout 120m
=== RUN   TestAccAWSSubnet_importBasic
--- PASS: TestAccAWSSubnet_importBasic (54.45s)
=== RUN   TestAccAWSSubnet_basic
--- PASS: TestAccAWSSubnet_basic (51.55s)
=== RUN   TestAccAWSSubnet_ipv6
--- PASS: TestAccAWSSubnet_ipv6 (124.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	230.024s

[grubernaut]

LGTM

[ghost]

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