provider/aws: Set aws_vpc ipv6 for associated only (#12899)
provider/aws: Set aws_vpc ipv6 for associated only (#12899)

Fixes: #12895

The AWS API returns both dissociated and associated IPv6 CIDRs. The UI
only returns the associated. Therefore, the assumption was made that we
would always take the 1st association in the set to use for state

We now loop over the set and look for the associated IPv6 CIDR before
using that in state

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSVpc_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/20 21:21:02 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSVpc_ -timeout 120m
=== RUN   TestAccAWSVpc_importBasic
--- PASS: TestAccAWSVpc_importBasic (65.91s)
=== RUN   TestAccAWSVpc_basic
--- PASS: TestAccAWSVpc_basic (50.88s)
=== RUN   TestAccAWSVpc_enableIpv6
--- PASS: TestAccAWSVpc_enableIpv6 (49.89s)
=== RUN   TestAccAWSVpc_dedicatedTenancy
--- PASS: TestAccAWSVpc_dedicatedTenancy (50.59s)
=== RUN   TestAccAWSVpc_tags
--- PASS: TestAccAWSVpc_tags (98.89s)
=== RUN   TestAccAWSVpc_update
--- PASS: TestAccAWSVpc_update (93.46s)
=== RUN   TestAccAWSVpc_bothDnsOptionsSet
--- PASS: TestAccAWSVpc_bothDnsOptionsSet (20.71s)
=== RUN   TestAccAWSVpc_DisabledDnsSupport
--- PASS: TestAccAWSVpc_DisabledDnsSupport (49.55s)
=== RUN   TestAccAWSVpc_classiclinkOptionSet
--- PASS: TestAccAWSVpc_classiclinkOptionSet (54.92s)
PASS
ok	github.com/hashicorp/terraform/builtin/providers/aws	534.829s
```
charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws