provider/aws: Add 'aws_default_subnet' resource

[ewbankkit]

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

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

[grubernaut]

Hey @ewbankkit, thanks for this!

Would you be able to paste the output of the acceptance test run? Thanks!

[stack72]

% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDefaultSubnet_'               ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/16 11:39:51 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDefaultSubnet_ -timeout 120m
=== RUN   TestAccAWSDefaultSubnet_basic
--- PASS: TestAccAWSDefaultSubnet_basic (22.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	22.021s

[ghost]

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