Remove Terraform duplication in `streaming/`

[NimJay]

• See this comment.
• The Terraform inside /streaming/kafka-strimzi/terraform is mostly identical to the Terraform added by https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/725.
• We should minimize this duplication.
• This would involve:
  • [ ] creating more generic (re-usable) Terraform (e.g., inside /streaming/kafka/terraform)
  • [ ] updating the two tutorials (Strimzi and Confluent) to use the new generic Terraform (/streaming/kafka/terraform)
  • [ ] removing unused Strimzi-specific and Confluent-sepecific Terraform from this repo