provider/aws: Set aws_vpc ipv6 for associated only

[stack72]

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

[grubernaut]

LGTM!

[dominics]

@stack72 A git bisect is showing this PR as the reason why Terraform suddenly wants to recreate my aws_vpc:

-/+ module.vpc.aws_vpc.vpc
    assign_generated_ipv6_cidr_block: "" => "false" (forces new resource)
    [...]

Let me know if you'd prefer me to open a new issue. Easy enough to workaround by adding assign_generated_ipv6_cidr_block = false but would seem to be a BC break.

[dominics]

Hmm, spoke too soon, and that workaround isn't working consistently for me: I've seen it plan to recreate the VPC even with assign_generated_ipv6_cidr_block = false explicitly set.

Edit: opened as #13035

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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