feat(module/vpn): VPN module for GCP VM-Series deployments

[pavelrn]

Description

Introducing VPN module for GCP VM-Series deployments. The module allows to deploy:

• VPN HA gateway with Cloud Router,
• multiple VPN instances (connections) where each instance is represented by remote gateway and multiple VPN tunnels.

PR closes Issue 215.

Motivation and Context

VPN module is required for building GCP to on-prem connectivity, for GCP to GCP connectivity.

How Has This Been Tested?

Terraform plan/apply using example VPN configuration.

Screenshots (if appropriate)

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.

[horiagunica]

I think the folder structure should be aligned with our current structure :

• Move example to examples folder
• I can see there is an inherited module - maybe it would be a good idea to flatten that module altogether and have them all in a single main.tf file.

[pavelrn]

@horiagunica

• Move example to examples folder

The reason why I didn't place the example into main examples folder is that the module itself is NOT a VM-Series example. I still believe that VPN module example doesn't belong to main examples folder.

• I can see there is an inherited module - maybe it would be a good idea to flatten that module altogether and have them all in a single main.tf file.

The reason why I placed vpn_instance in vpn module is that because
-vpn_instance is called from vpn and
-vpn_instance is not supposed to be used (will not be used) without vpn.
I still believe we should place vpn_instance inside vpn not to expose the modules that are not supposed to be used as external.

[pavelrn]

To align module structure with existing modules I decided to flatten VPN module (remove submodule).
Example code moved in the module's README

Issues has been fixed