Refactor terraform build/examples, remove heavy duplication

[zmerlynn]

Is your feature request related to a problem? Please describe.

There are three modules in the tree that reference install/terraform/modules/gke:

• build/terraform/gke
• build/terraform/e2e
• example/terraform-submodules/gke

Additionally, there are many make rules associated with these. It's not clear we need so many ways to access the same content, and some of it I feel has bit-rotted. The make rules in particular don't behave very well for me in Docker and we should consider just eliminating them and just requiring a terraform install.

Describe the solution you'd like

• A clear and concise description of the purpose of each terraform modules
• Possibly continuous testing of terraform in Docker if we choose to continue that

Additional context

Came up in review of #2912 - it's not at all clear which things we need make rules for.

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[markmandel]

The only things I can think of doing that are nice would be:

• Move the contents of build/includes/terraform.mk into build/includes/google-cloud.mk to make it easier to find.
• Double check we don't use gcloud-terraform-install anywhere, and if not delete it.

That's the only things I can find.