provider/aws: Improved the SNS topic subscription protocols validation

[Ninir]

Reasonning for this change

Partially fixes #10697

Acceptance tests

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSSNSTopicSubscription_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/12/13 17:53:20 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSSNSTopicSubscription_basic -timeout 120m
=== RUN   TestAccAWSSNSTopicSubscription_basic
--- PASS: TestAccAWSSNSTopicSubscription_basic (22.24s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	22.278s

[catsby]

Thank you @Ninir !

[ghost]

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