Set up submodule structure

[Jberlinsky]

Per conversation in #51 (review) and offline:

• Moves root Terraform configuration to ./autogen
• Creates submodule structure in ./modules and creates a public-cluster module

[aaron-lane]

A few small changes requested but otherwise looks good.

[morgante]

I think I might have been unclear on expectations here.

We should not be including complex Terraform-specific logic. Nor should we be causing a backwards-incompatible change to the root module.

Instead, this should be a pretty small and simple change:

1. Copying all the root files into an autogen/ folder and making them into templates. We can use jinja2 or Go for templating (open to alternative suggestions).
2. A very simple script which copies the files from the autogen folder into the specific modules. To start we will have just one: public. Keep the private in a follow-up PR.

To be clear, for this PR, we should avoid any symlinks.

[Jberlinsky]

@morgante To be clear, the first iteration of this will be template generation from ./autogen into / (relative to the repository root), to avoid changing main.tf and triggering a state migration. Future PRs will generate additional module(s) into ./modules/. Is that your understanding?

[morgante]

@Jberlinsky Yes, the addition of the private module can happen in a later PR.

[morgante]

We should also make sure the generated files include a header explaining they are autogenerated (and pointing to the autogen folder).

[morgante]

LGTM, merge once CI is green.