Add submodule and tests for private clusters

[Jberlinsky]

No description provided.

[adrienthebo]

Because we're autogenerating a bunch of code this changeset is pretty hefty. Are there specific areas you want reviewed more closely?

In addition there are some failing tests that look like they're still under development - do you want additional review at this time or do you want me to check back in a few days?

[Jberlinsky]

@adrienthebo Thanks for checking in, and apologies for the delay. This PR is now in a workable state, and is ready for review.

This is indeed a hefty change, due to the consolidation of logic in autogen/. The particularly relevant bits of this PR are the changes in autogen/cluster_regional.tf and autogen/cluster_zonal.tf, as they template out the major differences between the public and private cluster modules. I also added two test cases (suffixed with _private) to test such clusters.

[adrienthebo]

While there's a large diff the main changes in this PR just deal with templating out a few resource variables. Changes look good, test coverage looks solid, let's merge this.

[Jberlinsky]

Thanks @adrienthebo!

@aaron-lane could you kindly dismiss your review (or re-review if you're so inclined) so I can merge this?

[adrienthebo]

Verified that this change does not cause any Terraform state changes when upgraded from master.