Fixes: #13035
It was pointed out in the issue that the addition of a new parameter
with a default value AND a ForceNew: true is causing Terraform to try
and recreate the VPC
This PR migrates the state to add the default value of false for `assign_generated_ipv6_cidr_block`
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAWSVpcMigrateState'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/24 12:51:41 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAWSVpcMigrateState -timeout 120m
=== RUN TestAWSVpcMigrateState
2017/03/24 12:52:26 [INFO] Found AWS VPC State v0; migrating to v1
2017/03/24 12:52:26 [DEBUG] Attributes before migration: map[string]string{"assign_generated_ipv6_cidr_block":"true"}
2017/03/24 12:52:26 [DEBUG] Attributes after migration: map[string]string{"assign_generated_ipv6_cidr_block":"false"}
2017/03/24 12:52:26 [INFO] Found AWS VPC State v0; migrating to v1
2017/03/24 12:52:26 [DEBUG] Attributes before migration: map[string]string{}
2017/03/24 12:52:26 [DEBUG] Attributes after migration: map[string]string{"assign_generated_ipv6_cidr_block":"false"}
--- PASS: TestAWSVpcMigrateState (0.00s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 0.024s
```
4fe7ee16e6