provider/aws: Allow AWS Subnet to change IPv6 CIDR Block without ForceNew

[stack72]

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

[grubernaut]

LGTM

[ghost]

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