provider/aws: Fix reattachment of VPC to VPN gateway.

[modax]

When VPC is detached from VPN gateway, its VpcAttachment stays in place
just with state changed to "detached". Since terraform was not checking
attachment state, it used to think VPC gateway was still attached.

[catsby]

LGTM, thanks!

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSVpnGateway_ -timeout 120m
=== RUN   TestAccAWSVpnGateway_basic
--- PASS: TestAccAWSVpnGateway_basic (41.66s)
=== RUN   TestAccAWSVpnGateway_reattach
--- PASS: TestAccAWSVpnGateway_reattach (50.44s)
=== RUN   TestAccAWSVpnGateway_delete
--- PASS: TestAccAWSVpnGateway_delete (33.76s)
=== RUN   TestAccAWSVpnGateway_tags
--- PASS: TestAccAWSVpnGateway_tags (36.12s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    162.003s
Test:

[ghost]

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