Fixes: #13829
When IPv6 support was added to subnets, we added a new parameter that
had a default value. This means that users are experiencing unexpected
changes in their configuration
We need a schema migration in place to make sure this isn't the case for
the users who have not upgraded yet
```
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/23 10:36:43 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAWSSubnetMigrateState -timeout 120m
=== RUN TestAWSSubnetMigrateState
2017/04/23 10:37:27 [INFO] Found AWS Subnet State v0; migrating to v1
2017/04/23 10:37:27 [DEBUG] Attributes before migration: map[string]string{}
2017/04/23 10:37:27 [DEBUG] Attributes after migration: map[string]string{"assign_ipv6_address_on_creation":"false"}
--- PASS: TestAWSSubnetMigrateState (0.00s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 0.021s
```
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