provider/aws: Allow IPv6/IPv4 addresses to coexist

[bryanburgers]

Fix an issue when trying to get a public IPv4 address and a public IPv6
address that results in the following error:

Error launching source instance: InvalidParameterCombination:
Network interfaces and an instance-level IPv6 address count may not
be specified on the same request

To fix, in situations where we want a IPv6 addresses AND we need to
manually specify network interfaces on the instance, create the IPv6
addresses on the network interface that we're creating rather than on
the instance itself.

Fixes #13250

[bryanburgers]

I was unable to figure out how to get make test TEST=./builtin/providers/aws to work properly on my computer – even on master without my code – so I'm not highly confident in my acceptance test. However I did add an acceptance test and Travis succeeded. Maybe if I can get some other eyes on it?

[bryanburgers]

@stack72 You seem like a person that knows the AWS code well. Can you give me your thoughts on this PR?

[bryanburgers]

@mbfrahry @grubernaut I rebased this onto master to bring it up-to-date again. Can I get one of you to give this a review?

[grubernaut]

Hey @bryanburgers, thanks for the contribution!

In order to run acceptance tests, the testacc makefile target needs to be used:

$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_ipv6_supportAddressCountWithIpv4"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/31 10:03:40 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_ipv6_supportAddressCountWithIpv4 -timeout 120m
=== RUN   TestAccAWSInstance_ipv6_supportAddressCountWithIpv4
--- PASS: TestAccAWSInstance_ipv6_supportAddressCountWithIpv4 (144.70s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    144.724s

[grubernaut]

LGTM, thanks!

[ghost]

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