Fixes: #13588
It was pointed out in #13588 that we don't need to ForceNew on a change
of IPv6 CIDR block. The logic I decided to implement here was to
disassociate then associate. We should only be able to be associated to
1 IPv6 CIDR block at once. This feels like a risky move. We can
disassociate and then error on the associate. This would leave us in a
situation where we have no IPv6 CIDR block associated
The alternative here would be that the failure of association, triggers
a reassociation with the old IPv6 CIDR block
I added a test to make sure that the subnet Ids don't change as the ipv6
block changes. Before removing the ForceNew from the ipv6_cidr_block,
the test results in the following:
```
=== RUN TestAccAWSSubnet_ipv6
--- FAIL: TestAccAWSSubnet_ipv6 (92.09s)
resource_aws_subnet_test.go:105: Expected SubnetIDs not to change, but both got before: subnet-0d2b6a6a and after: subnet-742c6d13
```
After the removal of ForceNew, the test result looks as follows:
```
=== RUN TestAccAWSSubnet_ipv6
--- PASS: TestAccAWSSubnet_ipv6 (188.34s)
```
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSSubnet_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/24 21:26:36 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 (85.63s)
=== RUN TestAccAWSSubnet_basic
--- PASS: TestAccAWSSubnet_basic (80.28s)
=== RUN TestAccAWSSubnet_ipv6
--- PASS: TestAccAWSSubnet_ipv6 (188.34s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 354.283s
```
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