provider/aws: Fix SG update on instance with multiple network interfaces

[grubernaut]

With an EC2 instance that only had a single network interface, the primary interface, the Update function would call ModifyInstanceAttribute() on the target instance. This would only work if there was a single network interface attached to the EC2 instance. If, however, a secondary network interface was attached to the instance, the ModifyInstanceAttribute() API call would fail with the following error message:

There are multiple interfaces attached to instance 'i-XXXXX'. Please specify an interface ID for the operation instead.

After this changeset, modifying instance security groups now makes the correct call to ModifyNetworkInterfaceAttribute() in order to modify the list of security groups on the primary network interface, as initially configured during the instances creation.

This change is also safe from an instance that has a non-default primary network interface, as the instance attribute vpc_security_group_ids conflicts with the new network_interface attribute.

Test Output:

$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_addSecurityGroupNetworkInterface"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/08 17:52:42 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_addSecurityGroupNetworkInterface -timeout 120m
=== RUN   TestAccAWSInstance_addSecurityGroupNetworkInterface
--- PASS: TestAccAWSInstance_addSecurityGroupNetworkInterface (327.75s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    327.756s

Fixes: #3205, #7635
Related to: #7711

[catsby]

👍

[ghost]

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