provider/aws: Add support for api_gateway_stage

[radeksimko]

Related to #6972

This resource (intentionally) doesn't allow changing method settings - e.g. cloudwatch metrics or logging - that's because we don't have a very good support for maps with complex values and secondly we want to be able to change method settings for stages which were created as part of aws_api_gateway_deployment, not just for new stages.

I'm will submit a separate PR for aws_api_gateway_method_settings which satisfies the mentioned need and that should eventually close #6972

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSAPIGatewayStage_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/11 11:50:11 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSAPIGatewayStage_ -timeout 120m
=== RUN   TestAccAWSAPIGatewayStage_basic
--- PASS: TestAccAWSAPIGatewayStage_basic (539.63s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	539.659s

[grubernaut]

LGTM!

[ghost]

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