resource/aws: Add 'aws_default_vpc' resource

[ewbankkit]

Provides a resource to manage the default AWS VPC in the current region.
Acceptance tests:

make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccAWSDefaultVpc_'

[ewbankkit]

• Rebased to remove merge conflicts
• Added example to documentation

[ewbankkit]

Removing WIP.

[ewbankkit]

Rebased and added support for IPv6 attributes.

[ewbankkit]

Rebased and aligned acceptance tests with recent changes to aws_vpc resource.
Acceptance test output:

$ make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccAWSDefaultVpc_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 20:00:53 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccAWSDefaultVpc_ -timeout 120m
=== RUN   TestAccAWSDefaultVpc_basic
--- PASS: TestAccAWSDefaultVpc_basic (22.36s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	22.366s

[ewbankkit]

@stack72 Any chance of a review on this one? Thanks

[grubernaut]

LGTM, thanks!

[ghost]

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