Generate multi node spk template enhanced functionality

[NathanielRose]

As a: Developer

I want: to generate Terraform files for a multi-cluster based on parent-leaf definitions in parallel

So that: Accelerate the generation of my spk template across multiple deployment regions in one command.

Describe the solution you'd like: This is an extension to #721

Acceptance Criteria: Upon generation, spk infra generate (e.g. spk infra generate -p discovery-service) should:

• [ ] generate all leaf subdirectories to 1 level with a partial config implementation.
• [ ] spk infra generate -p discovery-service/east should generate east leaf definition with discover-service parent partial config.
• [ ] Within the project folder, spk infra generate should generate all leaf subdirectories to 1 level with a partial config implementation.

Additional context: https://github.com/CatalystCode/bedrock-end-to-end-dx#generating-deployable-cluster-terraform-scripts

Does this require updates to documentation?: Yes

[andrebriggs]

@NathanielRose @yradsmikham is this still needed?

[yradsmikham]

@andrebriggs Yes! This would be a nice enhancement to spk infra generate.

[andrebriggs]

@NathanielRose @yradsmikham is the main benefit that we are able to generate multiple sub directories in parallel? If so, have we measured how much of a time savings this would be?

[yradsmikham]

@andrebriggs, that is correct. I measure this as something between a S to a M.