provider/aws: Added conflictsWith for API GW Domain Name certificate_arn

[Ninir]

Description

This follows #12592 (comment) to add the ConflictsWith attribute, since you can only choose to have your certificate or an ACM one.

Tests

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSAPIGatewayDomainName_' 
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/13 15:59:40 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSAPIGatewayDomainName_ -timeout 120m
=== RUN   TestAccAWSAPIGatewayDomainName_basic
--- PASS: TestAccAWSAPIGatewayDomainName_basic (62.44s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	62.544s

[stack72]

LGTM!

[ghost]

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