Enable hub & spoke transitivity via gateway VMs

[drebes]

This enables transitivity between spokes using regional MIGs that act as transitivity gateways.

Feature gated by enable_hub_and_spoke_transitivity which depends on enable_hub_and_spoke.

Just tested locally, let me know what's needed for integration testing.

[bharathkkb]

Thanks for the PR @drebes
I have a fix #323 for the check_terraform error reported by the bot, but we should fix the others.
We should also update the cidr ranges being asserted in the tests here.

[rjerrems]

looks like there are still a few linting errors @drebes - if you would like to run this quickly locally, you can do so with this command make docker_test_lint

[bharathkkb]

Overall LGTM

• We can add a EXCLUDE_LINT_DIRS env var set to \./3-networks/modules/transitivity/assets to work around License header checker fails for Dockerfile and yaml GoogleCloudPlatform/cloud-foundation-toolkit#870. More info here. We will need to add this env var to the makefile and the cloudbuild lint pipeline.

• For integration tests we can enable enable_hub_and_spoke_transitivity in one of the network env fixtures and assert that the MIGs and ILBs are present. Is there a way to test the transitivity itself?

[drebes]

@bharathkkb done with the lint tests and rebased. I want to start looking into the integration tests now but I have zero experience with kitchen. Can you provide me a link to a similar PR where tests to new network related features were added? It's not clear to me where the tests set the new variables in neither #298 nor #345

[rjerrems]

LGTM - I think we can probably add the integration tests in a subsequent PR @drebes ?

cc @bharathkkb