provider/aws: Update ElasticTranscoder to allow empty notifications, removing notifications, etc

[catsby]

Allows users to supply empty ("") notifications for ElasticTranscoder, which amounts to unsetting them.

The current implementation doesn't allow you to remove any previously set ones, or omit any at all.

Adds acceptance test TestAccAWSElasticTranscoderPipeline_notifications which will fail on master.

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSElasticTranscoderPipeline_ -timeout 120m
=== RUN   TestAccAWSElasticTranscoderPipeline_basic
--- PASS: TestAccAWSElasticTranscoderPipeline_basic (24.10s)
=== RUN   TestAccAWSElasticTranscoderPipeline_notifications
--- PASS: TestAccAWSElasticTranscoderPipeline_notifications (35.44s) <-- new 
=== RUN   TestAccAWSElasticTranscoderPipeline_withContentConfig
--- PASS: TestAccAWSElasticTranscoderPipeline_withContentConfig (35.36s)
=== RUN   TestAccAWSElasticTranscoderPipeline_withPermissions
--- PASS: TestAccAWSElasticTranscoderPipeline_withPermissions (22.11s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    117.039s
Test:

Fixes #8163

[mitchellh]

Great, LGTM

[ghost]

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