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
```
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