Remove Terraform duplication in streaming/

[NimJay]

• See this comment.
• The Terraform inside /streaming/kafka-strimzi/terraform is mostly identical to the Terraform added by feat: samples for Confluent operator tutorial #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

[NimJay]

We should consider creating a Terraform module (or modules) shared repo-wide.
More Terraform (similar to existing Terraform) was introduced via https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/1487/files.