provider/aws: Added API Gateway integration update

[Ninir]

Description

This adds the update part to the API Gateway integration.

As per the documentation, everything excepted request_templates & request_parameters can't be added nor removed, for the attributes we have at time.
As they are not marked as Required, we need to flag them to be recreated on change, thus using the ForceNew attribute.

I reworked the acc tests so that they are clearer and handle this workflow, since the API to Create is different from the Update one (Series of PatchOperations):

1. Creation of resources using the
2. Update: Removal + Additions + Replaces
3. Removal & state checks

Related issues

• aws_api_gateway_integration resources not getting updated properly. #12233
• aws_api_gateway_integration request_parameters JSON?! #10223

Tests

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSAPIGatewayIntegration_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/31 17:51:35 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSAPIGatewayIntegration_basic -timeout 120m
=== RUN   TestAccAWSAPIGatewayIntegration_basic
--- PASS: TestAccAWSAPIGatewayIntegration_basic (96.88s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	96.913s

[Ninir]

This fixes the second issue

[stack72]

This is fantastic! Thanks so much for taking the time to work on this - LGTM!

[ghost]

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